MikeOxon

At the time it was the norm for express engines to be 'single wheelers' - probably because really accurate alignment of coupled wheels for fast running was still difficult. The GWR persisted with 'singles' later than most and built rather too many late in the 19th century.

My original model has conventional bogie arrangements, with the Tenshodo drive unit in the rear one. It even copes with my small radius curves!.

Almost 10 years ago, I wrote a post about Dean’s experimental 4-2-4 tank engine , which made a brief appearance in 1882 before being hurriedly rebuilt as a more conventional 2-2-2 tender engine. Very little information has survived about the original engine, except that it had a chronic inability to stay on the track. With so little prototype information available – and even less that could be considered reliable – I felt justified in taking considerable liberties in the design of my model. The most glaring divergence from received opinion is my arrangement of the bogies, with the longer one at the back. I arrived at this decision after considering the layout of the rebuilt 2-2-2 version, which indicated that the outside Stephenson valve gear could not fit, if the longer bogie were at the front. I illustrate this point in the following diagram: Bogie arrangements compared to photo of Rebuilt No.9 Other aspects of my model that are entirely fanciful are the extended cab roof and the decorative ‘fake’ wheel arch. So the following photo is my own interpretation, which may or may not have some similarity to Dean’s prototype. My representation of Dean’s 4-2-4T at North Leigh Station More information about the construction of my model, which was built by traditional methods, using brass sheet, and was powered by a Tenshodo SPUD motor in the rear bogie, was given in my original post . Coming to more recent times, the extraordinary 4-2-4 tanks that were designed by Pearson for the Bristol and Exeter Railway have entered my sphere of interest. I have modelled these recently, as described in my Broad Gauge blog . One fact that has emerged is that two examples of a version of the Pearson engines, with smaller 7’ 6” diameter driving wheels, survived into GWR ownership, when they were numbered 2005 and 2006. GWR No. 2005, which was built at Bristol in 1862, remained in its original condition until broken up in about 1887. The point here is that this date is after Dean designed his standard gauge version! Furthermore, Ahrons, in “Locomotive and Train Working in the Latter Part of the Nineteenth Century”, vol.4, reports seeing number 2005 frequently at Swindon and Bristol shed. Taken together, these facts suggest that there may have been a closer relationship between Dean’s 4-2-4T and the Pearson 7’ 6” GWR No.2005 than has been recognised previously. In order to examine this relationship further, I decided to create 3D models of both No,2005 (broad gauge) and No.9 (standard gauge), so that I could place them side-by-side and consider the similarities and differences. In this blog, I shall describe my creation of a 3D model of No.9, while I shall tackle the other engine in my Broad Gauge blog. Creating a 3D Model of No.9 As I mentioned above, my existing model of No.9 was built by traditional methods, using brass sheet that I cut out by hand over paper templates. I still have the drawings, made using ‘Autosketch’ software, so I started by importing these drawings, as a ‘canvas’, into ‘Fusion 360’. I then followed my usual process of extruding the various components – boiler, firebox, frames, etc - from the drawings, to create 3D structures. I have previously described my methods in a blog post about creating a 3D model of a GWR ‘Sir Daniel’ class engine. For my current model, the initial layout of the components looked as shown below: Outlines of my Model Components over ‘Canvas’ in Fusion 360’ I added various details such as the outside Stephenson valve gear and the bogie side frames, to bring the 3D model up to a similar stage of detail as my brass model and then rendered the computer model in appropriate colours. After taking a screen shot of the 3D model, I added some more livery details in 'Photoshop' to give ‘character’ to the result. Remember that most of this comes from my own imagination, as we know very little about how the prototype was finished. I have tried to make it look like a ‘prestige’ express engine, which was apparently the original intention. My 3D model of No.9, rendered in ‘Fusion 360’ If the prototype really looked anything like this, I can understand why David Joy recorded in his diary, following a visit to Swindon in 1882 “I saw all about a mighty 'single' tank engine Dean and Charlton were building—8 ft-single and double 4 ft. - wheel bogies at each end. I saw drawings and all, and she looked a beauty. She was intended to do Paddington to Swindon in 2 min. under time," Comparison with Pearson Broad Gauge 4-2-4T I have described my 3D modelling of Pearson's engine in my Broad Gauge blog. Pearson (broad gauge) and Dean (standard gauge) 4-2-4T Engines compared Looking at these two together, I think it is fairly obvious why the Dean engine had difficulty in staying on the track! With Dean’s inflexible bogie design and the excessive overall length, the lateral forces on any slight curvature must have been considerable, not helped by the large masses of water sloshing about in the long side tanks. Dean was faced with several problems. He wanted to emulate the boiler capacity of Gooch’s 8 ft. ‘singles’, which would soon have to be replaced, so he had to increase the length to compensate for loss of width possible on a broad gauge engine. According to the RCTS booklet Part Two, “The domeless boiler was itself a. phenomenon, for it was one of the first in this country to be made in two rings and withal had a barrel length of 11ft. 6in.. not destined to be repeated for another ten years.”. The firebox also had to be lengthened, to maintain a grate area comparable with the wide firebox that was possible on the broad gauge. Another problem was how to accommodate large diameter cylinders, like those used on the broad gauge, together with their associated valves and steam chests within the narrower space between the frames. He tried placing the valves above the cylinder, operating them through rocking shafts from outside Stephenson valve gear. Most authorities agree that this engine was a complete disaster and must have been a considerable embarrassment to Dean - it’s not surprising that he didn’t want it talked about too much!. But he got over it and eventually came up with his own ‘singles’, which moved the valves below the cylinders in the ‘Stroudley’ arrangement and provided a much improved design of front bogie (after a pair of leading wheels proved insufficient) to keep the machine on the track. The long side tanks had to go and greater water capacity was obtained from a lengthened 6-wheel tender It is possible that the two engines did actually come together at Swindon, since No.2005 was still around and, according to Ahrons, a frequent visitor to Swindon. Dean 4-2-4T meets Pearson 4-2-4T Mike

After dissecting the workings of the extraordinary 9ft. Pearson 4-2-4T engines in my previous four posts , I was interested to examine how these engines compared with William Dean’s later attempt to create something similar for the standard gauge. To make the comparison on as level a playing field as possible, I looked up information on the slightly later Pearson engines fitted with smaller 7’ 6” driving wheels – similar to those on Dean’s standard gauge engine. I have previously modelled the Dean engine, as described in my Pre-Grouping blog . Thee were eight of the original Pearson 4-2-4T design, with 9 ft. driving wheels, all built by Rothwell & Co. and delivered in 1853-4. There must have been difficulties arising from their novel features, as they were all rebuilt, with the usual form of inside frames and conventional springs, in 1868-70. In between these batches, two more 4-2-4T engines were built for the Bristol & Exeter Railway (B&ER) in Bristol. These were similar to the 9 ft. engines but with smaller driving wheels of 7’ 6” diameter. No. 29 was delivered in September, 1859 and No. 12 in April, 1862. These engines survived into GWR ownership and, as GWR No. 2005, the former No.12 remained in its original condition until broken up in about 1887. Ahrons, in “Locomotive and Train Working in the Latter Part of the Nineteenth Century”,Vol. Four, 1953, reports seeing No. 2005 frequently at Swindon and Bristol sheds. GWR No.2005 formerly B&ER No.12, built 1862 and broken up 1887 Taken together, these facts suggest that there may have been a closer relationship than has been recognised previously between Dean’s standard gauge 4-2-4T and the Pearson broad gauge 7’ 6”, GWR No.2005. Since I have already created a model of the Dean engine , I wanted to see how it compared with No.2005, by placing two models ‘side by side’. Creating a Model of B&ER No.12 (GWR No.2005) No.12 has been described as being similar to the earlier 9 ft. singles, although many of the ‘exotic’ features of the earlier engines had already been abandoned by then, even before the original engines were re-built. I found a fairly detailed description of the smaller engines, including an outline drawing, in ‘The Locomotive Magazine’, Vol . III. No. 36. Dec.1898 According to ‘The Locomotive Magazine’: “Their driving wheels were only 7ft. 6in. Diameter…. The diameter of the bogie wheels was 4ft, and the total wheel base was 25ft. 2in, the leading bogie having a base of 5ft. 6in., whilst that of the trailing bogie was 5ft. 9in., the driving wheels were 9ft. 4in. behind the leading bogie centre, and 10ft. 3in. in advance of the trailing. The boiler was 9ft. 9in. long, its maximum external diameter being 4ft. 2in., and the height of its centre line above the rail level 6ft. 11in.” This information was adequate for me to create a 3D model, which I based on my existing model of one of the 9 ft. engines. It was actually an easier modelling task, since these engines had conventional inside frames. Some peculiar featured remained, however, such as the water tank underneath the ashpan! Following my usual method, I created the boiler-smokebox-firebox assembly by reference to the above drawing, imported into ‘Fusion 360’ as a ‘canvas’. In addition to the driving wheels being smaller, the boiler was 1 ft. shorter than on the earlier engines. I used the ‘Move’ tool in ‘Fusion 360’ to modify faces of the original models of the 9 ft. engines, so as to match the profiles taken from the drawing. Creating 3D Boiler Assy with reference to Drawing I had to make new driving wheels, by my usual method, but re-used the bogie wheels and then assembled all the wheels around a new pair of full-length inside frames Creating 3D Chassis with reference to Drawing I took advantage of the way in which objects can pass through one another in the virtual world, by creating the two cylinders as complete ‘bodies’ that were then largely enclosed within the smokebox with parts of the sides protruding. The coke bunker only needed slight modification and the chimney and safety valve cover had to be re-profiled. One item which I have not modelled before was the curved handrail, which is such a prominent feature as it loops above the driving wheel splasher. This feature is easy to create in ‘Fusion 360’ by using the ‘Sweep’ tool. The path to be taken by the handrail is first created as a sketch, using the ‘arc’ and ‘line’ tools. Next the circular profile of the rail has to be created in a perpendicular plane. The ‘Sweep’ tool then causes the ‘profile’ to be extruded along the ‘path’, as illustrated below. Using the ‘Sweep’ tool to create a curved handrail in ‘Fusion 360’ So, it needed surprisingly little re-work before I had another model, representing the smaller-wheeled version of Pearson’s 4-2-4 tanks. My 3D model of Pearson’s 7’ 6” version of his 4-2-4T in GWR livery There’s a lot more detailing that could be added but I think this gives a good impression of the overall appearance of the real locomotive. Comparison with Dean’s standard-gauge version In parallel with developing this 3D model, I have also re-modelled the Dean standard gauge 4-2-4T in ‘Fusion 360’, so that I could place both versions together, to make some direct visual comparisons: I describe creating my 3D standard gauge model in in my Pre-Grouping blog. Pearson (broad gauge) and Dean (standard gauge) 4-2-4T Engines compared Looking at these two together, I think it is fairly obvious why the Dean engine had difficulty in staying on the track! With Dean’s inflexible bogie design and the excessive overall length, the lateral forces on any slight curvature must have been considerable, not helped by the large masses of water sloshing about in the long side tanks. It is possible that the two engines did actually come together at Swindon, since No.2005 was still around when No.9 was built and, according to Ahrons, a frequent visitor to Swindon. Dean 4-2-4T meets Pearson 4-2-4T Mike

Perhaps we need a glossary of technical terms. Off-hand I recall ‘dismals’ and ‘coal cart gauge’ but I feel sure that there are many others 😃

Sorry to hear about your second experience of Covid but it sounds as though you are on the mend. That's a splendid BG photo you've found, although it does show how 'old fashioned' the engine was looking in the last days of the BG - a final glimpse of polished brass and large single drivers before the rot set in 😀 Mike

I shall have to make some simplifications but I expect to print this as a 'mantlepiece' ornament.

I ended Part Three with the prospect of modelling the many rods and brackets on the underside looming over me. I had intended to write more at that time but found myself struggling to understand how various parts of the engine fitted together. I think all the ‘easy’ bits have now been done, so I could no longer avoid the complex underpinnings. To gain an overview, I ‘mirrored’ one half of the split plan-view from ‘The Engineer’ and then colour-coded various elements – blue for frames, orange for crankshafts, green for valve gear, and red for wheel bearings. I made a couple of ‘corrections’ to the ‘mirror’ process by moving the cranks on one side to represent ‘quartering’. I have repeated this plan as a ‘header’ to this entry. following its use in Part Three . My 3D model overlaid on ‘The Engineer’ plan view I was pleased to find more information, which helped me interpret the various drawings, in an article from ‘Engineering’, 11th Feb.1870 (reproduced in the Broad Gauge Society (BGS) journal ‘Broadsheet’ No.27, Spring 1992). Although the article refers to the ‘rebuilds’, some of the information appears to apply to the original engines as well. I quote: “…. There is also a centre stay for the crank axle fitted with adjustable wedges; this stay is bolted to transverse plate in front of the firebox which ties the frames and assists in supporting the stay; The eccentric sheaves are of cast iron, as are also their respective straps, these latter having cast on the half that receives the rod two ears which with a pin inserted vertically and eye in the eccentric rod make a lateral joint. The valve gear is of that class known as Gooch‘s stationary link. ... The valve spindles are. guided by a cast-iron bracket bolted to the plates which carry the bogie pin and unite the boiler barrel with the smoke-box tube plate; these brackets have each a flat bar of iron or steel fitted for the spindle crossheads to slide on; these crossheads being similar to the piston crossheads. The reversing shaft is carried by two brackets bolted to the bottom slide bars.” Gooch ‘Stationary Link’ Valve gear I then found a lot more useful information in articles by Douglas S Johnson, published in two issues of ‘Broadsheet’, Nos. 83 and 84 (2020), in which he described constructing a model the ‘hard way’, using nickel silver and brass. While very helpful, these articles also provoked great sighs of relief that I was using 3D computer modelling, rather than facing the problems raised by real model engineering. Modelling the ‘Motion’ As before, I have tried to follow a ‘line of least resistance’, so decided that the moving parts of the motion were the easiest components to understand and place in their appropriate locations. My hope was that the locations of the various supporting brackets would become more obvious once I had the moving parts in place. One of the great things about 3D modelling in a computer is that individual parts will stay where they are placed, as though on ‘sky hooks’! Sketch of Motion over ‘The Engineer’ Drawing I started with the main drive-shafts between the cylinders and the driving wheel cranks. The rods are simply cylinders, produced by extruding their cross-section drawings. I have simplified the cross head by extruding from a plan view and then set in place two slide bars, above and below the cross head. I show these parts above the ‘canvas’ which provided me with the overall dimensions. My representation of the main drive components These parts will form a static representation of the motion – fully working motion would need metal bars and bearings, which are not on my agenda at present. Because of their prominent locations, they are needed for completing the external appearance of my model. Side view of the Motion in place on my model I followed up by using similar methods to create the various components of the valve gear. I made the profile of the Gooch stationary link by tracing over the above sketch of the valve gear and then created the various rods by simple extrusions from sketches. After creating the various components individually, I moved them into their appropriate locations on one side of the engine and then ‘mirrored’ the whole lot to the other side. My layout of Valve gear components Next, I put the components into the context of the rest of the model (minus boiler and smokebox), to help me to determine where the various supporting structures need to be placed. Setting the Motion in the context of my Model Before I could get much further, I needed to develop a better understanding of how this engine ‘worked’. Overall Engine Structure In most engines, the driving wheels transmit the force needed to pull the train, through a pair of strong plate frames running the full length on each side of the engine. These are linked at the back to a strong drag bar running across the width of the engine and carrying the couplings to following vehicles. In this Pearson engine, the strong plate frames are notably absent. The design has been likened to a road-going Traction Engine but, although there are similarities, they are not the same. In a Traction Engine, the driving wheels are near the back and transmit their forces through a strong frame at the rear end, which carries the necessary draw gear. The boiler in such an engine is a forward extension from the ‘pulling part’ of the engine, carried at its forward end by the steerable front wheels. A different analogy can be found in Brunel’s design for his Chepstow Bridge, in which he took advantage of the considerable strength of an iron tube to transmit both compression and tension forces. In Pearson’s engine, it is the boiler that provides this key structural component, being connected to the central driving axle through the yoke spanning the top of the boiler. As a tank engine, the design was intended to work in both directions. When running forwards the boiler transmitted the driving force in turn to the firebox, through a transverse frame member, and then to the rectangular tank underneath the coal bunker. The rear coupling hook was bolted directly to the back of this tank, which acted as a box girder. For running backwards the forces were carried by two plates riveted to the lower sides of the boiler, which transmitted the forces to the cylinder casting and then by a short shaft to the front coupling. I should point out that the above is my own interpretation after spending several days looking at drawings. If those with more engineering expertise see it differently then I shall be pleased to be corrected. This method of conveying the main driving forces through the boiler would not be permitted now. The fact that even substantial plate frames were subject to cracking under stress, suggests what could happen to a pressurised boiler in similar circumstances. Modelling the Structure It took a lot of head-scratching and poring over drawings before, largely by trial and error, I worked out how everything fitted together. The drawings show a plethora of riveted plates, which took me some time before I could understand their functions and how they fitted within the overall context of the engine as a working vehicle. I’m not sure that I can now recall all the steps that I made (and an account would be very tedious anyway) but the outcome of all my deliberations is shown below. I started with the basic rectangular frame, described Ahrons as “only 8in. deep for the greater part of its length except at the driving hornblocks. An arrangement of angle plates, 2ft. deep, was fastened to the side of the fire-box and to the front of the well tank. From this point to the back buffer beam there was no frame at all.” Next, I had to understand the curved plate that can be seen in ‘The Engineer’ side elevation, extending from the back of the smokebox and riveted along the lower sides of the boiler. I determined that there were actually two of these plates attached on either side of the casting that carries the front bogie mount. Their purpose was, apparently, to transfer tractive forces from the boiler to the front coupling on the engine. I placed them on my model as shown below: Modelling the Front-end Boiler Brackets I could now place the ‘motion’ I described earlier into the context of these brackets and the rectangular frame, as shown below: Setting the motion within the inside frame I could now work out the arrangements for the centre bearing of the crank axle and its fore and aft attachments to the firebox and front well tank. Centre-bearing for Crank Axle (outer bearings not shown) It all looks so simple now – it’s hard to take in how long it took me to figure all this out from the drawings I have 🙂 Actually, when I put it all together, perhaps it doesn’t look quite so simple! Quite a step up from my previous modelling methods: My model of the ‘Works’ It’s rather a pity that almost all of this becomes invisible once the boiler and outer frames are in place 😒 I also find myself wondering how the real engine was erected, with so many ‘inter-dependent’ parts. My 3D model in ‘photographic grey’ There’s not even much to see from underneath because it’s hidden by the well tank. My 3D model viewed from below After rendering in ‘Fusion 360’ my model looks like this: My 3D model rendered in ‘Fusion 360’ You’d have to look at this rather carefully to spot any visible differences from my earlier renderings! Now that I’ve teased out most of the internal features, which has been an ‘interesting’ mental exercise, I shall have to return to considering the ‘cosmetic’ appearance. There’s still a lot to be done on the details, such as rivets, boiler bands, and so on … and on. Oh, and brake gear on the rear bogie. Enough for now Mike

The history of these engines has received extensive coverage in various issues of the BGS journal 'Broadsheet'. My fellow BGS member, Douglas S Johnson, built a model the hard way, using brass and nickel silver, and wrote a full description over two issues of the 'Broadsheet' in 2020. Concerning drawings, he wrote: "There is some dispute over the validity of some of the ‘original’ drawings - see ‘Nemesis’ in [Broadsheet] 54.30. This is understandable, as some ‘as-built’ drawings were issued by Swindon for the centenary exhibition, while others are B&ER drawings dated August 1852 — April 1857, together seeming to be the basis for the drawings printed in ‘The Engineer“ of December 1910. There may be no good reason to doubt the accuracy of these drawings, despite their various dates and provenance, as being a near true representation of how the locomotives first ran, or were intended to run.The photographs show some things otherwise. as should be expected given the rapid pace of development in that period." Douglas generously showed me copies of several of the early drawings, which were a great help to me in understanding some of the details of these engines. Mike

By the end of Part Two , I had modelled all the most visible parts of the engine and felt tempted to stop there but many of the peculiarities of these engines were below the platform, so I had to keep going ‘down there’. Photo by Snell of B&ER 4-2-4T No.42 Although I have collected quite a number of drawings and photos, there are still some difficulties in determining the layout of all the parts, especially since some drawings omit features and others show some profiles, without indicating their locations in three dimensions. Well Tanks I decided to start with the two well tanks, once below the boiler and the other below the coke bunker, since these are well displayed in the three-view illustrations from ‘The Engineer’ supplement, 1910, which I showed in Part Two. I sketched the profiles by using the ‘Rectangle’ tool in ‘Fusion 360’ to trace over the end elevation illustrations. I then extruded the profiles to the lengths indicated on the side elevations. These steps are shown below: Locating model well tanks against ‘The Engineer’ illustrations The above drawings show the internal bracing struts inside the tank under the bunker, which served to reinforce the mounting for the ball on which the rear bogie was pivoted. While I have not modelled these completely concealed structures, they provided me with useful guidance on the placement of similar-looking braces at the font-end of the engine. The drawings show that there were two upward extensions from the tank under the bunker, leading up to the filler caps. These structures also served to separate the crew footplate from the coke bunker itself, behind them. A tool box and brake handle were also placed above the footplate. According to Ahrons: “An arrangement of angle plates, 2ft. deep, was fastened to the side of the fire-box and to the front of the well tank. From this point to the back buffer beam there was no frame at all.” I sketched the outlines of the tank extensions and the toolbox by tracing over the plan view from ‘The Engineer’ and extruded upwards from the sketches to match the illustration of the elevation. The results are shown below: Coke Bunker with Toolbox and Tank Fillers Front Bogie Mountings Having secured the rear bogie on its ball and socket joint, it was time to turn to the front end. According to Ahrons: “The ball of the leading bogie was secured to the underside of the cylinders by means of a casting with wings, to which two horizontal tie rods were fastened ; the other ends of the latter were secured to the bogie side frames, and prevented the bogies from slewing round across the track.” I attempted to identify these features from the front-end elevation shown in ‘The Engineer’ illustrations. Front Elevation from ‘The Engineer’ I assume that the casting for the ball is the part I have coloured blue, while the ‘wings’ are the parts coloured orange. The tie-rods to the bogie frame can be seen extending outwards from pivots on these ‘wings’. Where, though, is the brace coloured red to be placed? It is shown crossing in front of the tie rods so, perhaps, as at the rear end, there were two braces – fore and aft of the tie rods. I have enhanced the relevant area from the photo of No.42 above: Detail from Snell’s photo of No.42 The photo clearly shows a reinforcing bracket on the bogie side fame and what looks like the end of a tie-rod just above the frame. It appears that there were bracing plates either side of the tie-rod, which may be what is represented on the front-elevation drawing. Unless anyone has any more information or I find another drawing, I have to go with this assumption. I created the following support structure by first tracing the profile of the ball and the casting immediately above it, followed by using the ‘Revolve’ tool to create a cylindrical ‘body’. Then I sketched ‘wings’ either side of the central body. For the bracket, I traced the front-elevation profile and extruded it, initially with a rectangular profile. I then used the ‘Cut’ tool across the extruded width to create the sloping sides seen in the photo above and the central slot through which the tie-bars pass. My result is shown below, with the component parts coloured as in the illustration above. I also show the completed model, assembled above the front bogie: My interpretation of the front bogie support frame With the well tanks and bogie attachments in place, the underside of my model now looks like this: My model underside with well tanks in place As an aside, I think this underside view demonstrates why Dean failed in his attempt to create a narrow (standard) gauge version of a 4-2-4 tank engine. There was no room for the large well tanks so he had to resort to large side tanks, which were a source of severe instability. I have previously modelled Dean’s experimental No.9, as described in my Pre-Grouping blog. There’s a lot more detail still to be added to the underside. To gain an overview, I ‘mirrored’ one half of the split plan-view from ‘The Engineer’ and then colour-coded various elements – blue for frames, orange for crankshafts, green for valve gear, and red for wheel bearings. I made a couple of ‘corrections’ to the ‘mirror’ process by moving the cranks on one side to represent ‘quartering’: My Colour-coded plan view, derived from ‘The Engineer’ illustration As I began to examine this underside view in conjunction with the various elevations, I realised that the complex array of brackets and plates was not going to be easy to unravel! On this engine, the motion is very visible in side views, so I cannot escape modelling its main features. It has become clear that it’s going to take me some time to work out how all these parts fitted together in three dimensions, so I’ve decided to take a break before starting on modelling the motion and various underpinnings. This will a new area for me, since I have neglected any detailed representation of the motion on the engines I have designed previously. Mike

As I discovered when I started designing brickwork for my coke kilns, these traditional crafts have many pitfalls for the unwary. I watched a few YouTube videos to learn the rudiments of bricklaying

A fascinating (and useful) 'walk though' the stages of assembling a kit model. Thank you.

Sadly, Brunel failed to recognised that it wasn't the rail gauge that was the problem but the loading gauge, which still hobbles British railways today. The Americans planned for larger trains from the start. Mike

On he drawing of the suspension from 'The Engineer', the tyre width is marked as 6" (15.24 cm)

Been there ;

I find the trouble is that once you know something should be there, you feel obliged to model it 🙂

I agree, it is very hard to produce anything other than an 'outline' from those drawings. Beware, I have also found several inaccuracies in those drawings, many of which emanate from 'The Locomotive Magazine'. I also find it difficult to interpret more detailed drawings, mainly in trying to work out how various parts were orientated in three dimensions. The underside is proving particularly difficult.

Thank you Chris - that's exactly what I am trying to do. I've started looking at the underside and it seems to have been an extraordinary assemblage of brackets with no proper frames at all. Quite a few early engines had 'structural' boilers. The 'Firefly' replica at Didcot had to include additional framing, to meet current regulations

I simply move and rotate the parts I have made and align them against my reference 'canvas'. I have hidden the rest of the model for clarity [NB I've added this clarification to the main text]: I rarely do any measurements or create offset planes unless I have to 🙂

In Part One , I wrote that “this engine had several very unusual features” and, in regard to building a model, “I had to start somewhere and, with so many peculiarities, it was hard to choose. As a ‘gentle introduction’, I decided to start with the two bogies.” I intend to continue, as far as possible, to follow a line of ‘least resistance’ but before going any further, I collected as much potentially useful information , photos, and drawings as I could. In his book ‘The British Steam Locomotive 1825 – 1925’ p.106, Ahrons noted that “no description of them, beyond the meagre details in Colburn's " Locomotive Engineering," page 73, has ever been given, and the following account of their constructional details may therefore be of interest.” He continued with quite an extensive article, providing many key dimensions and descriptions of the rubber suspension system. This suspension anticipated Alex Moulton’s work for the Austin Mini by more than a century! Ahron’s also referenced an article in ‘The Engineer’ supplement, 16 Dec 1910 , which provided various sectional views of the engine from which I could make a start. Using these drawings, I started by creating some of those major components for which I had already gained experience in modelling other engines. Ahrons gave the boiler dimensions as 10ft. 9in. long by 4ft. 0½ in. inside diameter, so I created a tube of length 43 mm, inside dia. 16 mm, with 1 mm wall thickness, which provided a good match to an ‘The Engineer’ drawing. I then added firebox and smokebox by tracing over the drawing and extruding as required, to create solid ‘bodies’ in ‘Fusion 360’. The results were as shown below: 3D model of Boiler Assy. Referenced from an illustration in ‘The Engineer’. The next ‘familiar’ item on the agenda was a pair of driving wheels. although these are a little larger than usual, at 36 mm diameter, and flangeless. The stages of my usual method are shown below: My steps in 3D modelling the Driving Wheels It always surprises me how rapidly something resembling an engine emerges, especially once the platform, chimney and safety valve housing have been added. The chimney and safety valve cover were created by my usual method of tracing the profile and then using the ‘Revolve’ tool to create the cylindrical ‘bodies’. The platform was a simple rectangular extrusion from the plan drawing, with the exception that small ‘humps’ had to be raised over each of the bogie wheels. Those early designers did nothing to make life easier for the workmen having to fabricate these shapes by hand! First impression of my evolving model That has completed most of the straightforward parts of this engine and it is now time to start tackling its (many) peculiarities! Outside ‘Frame’ According to Ahrons “The inside bearings were only 5in. long, and therefore additional outside bearings, 9in. long, were provided, the hornblocks of which were riveted to the triangular queen truss " frame," shown outside the driving wheel”. This ‘frame’ was my next subject for modelling. I created the truss and the outside rim of the splasher as a single ‘body’ in ‘Fusion 360’ by extruding from a drawing, as shown below: My sketch of the outside frame supporting the driving axle Most of the sketch was made by using the ‘three point arc’ tool in ‘Fusion 360’ and then I extruded the area coloured blue to form the frame. Next, I extruded the rectangle representing the axlebox, to house the outside bearing. I used another useful tool – the ‘offset’ tool – to create an outer rim around the frame and then ‘pushed’ the annulus (coloured yellow) backwards, to surround the driving wheel as the splasher. Yoke and Boiler Support Plates Now it was time to address two more peculiarities – the yoke across the top of the boiler and the curvaceous plates that supported the boiler from the platform. I created both these features by sketching over one of the front-elevation illustrations from ‘The Engineer’. Again, the most useful sketching tool was the ‘three point arc’ and I then extruded the outlined area to form plates of 0.5 mm thickness, as shown below: There were two identical ‘yoke’ plates in tandem (blue in my sketch) straddling the top of the boiler, with pivoted brackets between them, to carry the vertical suspension rods above the riving wheels. Two curved support plates (green) were placed, one on either side of the boiler, immediately ahead of the driving wheel splashers. Tracing the shapes of the Curved Plates around the Boiler I then move and rotate the parts I have made and align them against my reference 'canvas'. I have hidden the rest of the model for clarity: After producing models of these plates, I moved on to the suspension units for the driving axle. These were illustrated and described by Ahrons. I created my models of the suspension units by sketching the profile over the drawing from ‘The Engineer’ and then using the ‘Revolve’ tool in ‘Fusion 360’ to create the cylindrical rods and the brass pots that contain the india rubber ‘springs, as shown below: My extrusion of a suspension arm alongside ‘The Engineer’ Drawing Motion Plate The motion plate was, again, unusual in that its outer edges were shaped to follow similar curves to those of the boiler support plates. For details of their appearance, I had to turn to another drawing, shown in the Broad Gauge Society (BGS) journal ‘Broadsheet’ No.49 (Spring 2003). The original is one of those ‘split’ drawings, with halves of two different cross sections shown together. To help my visualisation of the engine, I 'mirrored' each half to provide two separate complete cross sections, although I cannot guarantee that all the details on the two sides were perfectly matched. Two cross-sections created from drawings in BGS ‘Broadsheet’ No.49 In my drawing ‘B’, I have shaded the split motion plate, which has square apertures for the connecting rods to pass through. I copied the outlines of these motion plates, as for the other boiler support plates, and extruded them to 0.5 mm thickness. The valves were placed between the cylinders and the valve rods are shown with supporting slide bars. There are many other unusual features in these drawings that I shall return to later, including the well tank suspended below the boiler on brackets from the motion plate. Cylinders The cylinders themselves were too long to fit within the very short smokebox, so they extended for about 1 foot forwards of the smokebox. They were connected across the width of the engine by a box, which enclosed most of the front end of the valve gear, although there were two tail rods protruding from the front face of this box. A photograph of the front end of No.42 also shows a small steam cock on the centre of his cover I constructed this box and the cylinder ends as a separate ‘body’, which I then fixed to the front of the smokebox. My model of the cylinder ‘front end’ That has completed all the main components needed for a ‘top-side’ view of the engine. I have added buffer beams, copied from a different Broad Gauge engine, and extruded the outer sides of the coke bunker. Although simple in external appearance, this bunker has many unusual internal features but I shall come to these later. In the meantime, my 3D model in ‘Fusion 360’ now looks as shown below: I think this model is beginning to capture something of the ‘presence’ that the original engines must had. I still have a lot of work to do on the underpinnings – especially the attachment points for the bogies and the well tanks but right now I feel it’s time to pause for the Easter break! Mike

I look forward to seeing what you've been doing since our previous correspondence Mike

Luxury! - steps and handrails. No sign of those in the original versions. Interesting that GWR removed the cab. Armstrong did not like cabs because he thought that fumes from the fire would make crews drowsy.

'interesting', as in "may you live in interesting times"🙂 Trying to form a good 'picture' in my mind from the various drawings I have is taxing me at present - it's hard to interpret some of the oddities. Those pots over the driving wheel are indeed rubber suspension - they are on both sides of each wheel and I'm still puzzling over how it all worked.

Perhaps Pearson felt that he could try out a few ideas on a branch-line engine without the risk of too much exposure if some things didn't work out. The more extraordinary features were removed when the engines were re-built. Ahrons wrote: "In 1868-73, after an average life of about sixteen years, all the eight original Rothwell engines were scrapped, a fact which shows that they left something to be desired. Four new engines in which practically nothing of the originals remained, were built at Bristol in their place." The date on The Engineer illustration I showed is incorrect for the original engines (as shown), which actually dated from 1853.

I need to add the 'pots' for them behind the wheels, as on this drawing: I keep finding new things as I pore over various drawings. These engines really were peculiar.