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Rhymney Railway M Class



The last of the outside framed classes had been delivered to the Rhymney in 1900, and from then on the locomotives took on a much more modern appearance. The first were what was later to be called the M class, and the  detailed design is usually credited to Robert Stephenson & Co. All subsequent locomotives for the Rhymney, other than a pair of locomotives which started life as railmotor units, bore a distinct family resemblance. These locomotives form a complicated and rather incestuous group, since around 17 variations on the 0-6-2 theme can be identified, all sharing a common 7ft 3in + 8ft 0in + 6ft 0in wheelbase which was also seen on similar locomotives for other lines and even the GWR's 56xx class. Within that there are three basic themes: 4'6 wheels with larger boilers (M, R), 5ft wheel with a smaller firebox (P), and 4'4.5 in wheel also with the smaller boiler (A). There were subclasses with round top and belpaire firebox boilers, and some experiments were made with boilers swapped between classes. When the GWR introduced their own boilers the Ms and Rs received Std 2s, and the Ps and As Std 10. It's also helpful to consider the S and S1 class 0-6-0T when looking at the RR locomotive development. The actual construction order was M, R, S, P, A, A1, P1, S1, AR/AP, and its helpful to keep this in mind tracing the development between classes.

The original boilers on the Ms, which had Belpaire fireboxes, were considered unsatisfactory, and by the grouping all were running boilers of the design first used by the R class. The GWR tended to treat the Ms as one class with the later Rs, albeit with subclasses represented with different diagrams. They were numbered 33 and 47-51.


This first sketch represents what was temporarily known as the Mr Class, being the M class fitted with the R class boiler. Once all had been converted the r disappeared again.





The Ms received R class cylinders in the 1930s. One was rebuilt with a Standard 2 boiler and GWR style cab, but otherwise they had few changes beyond safety valves and larger bunkers. The second sketch is intended to show the one M, no 47, that received a GWR boiler - a standard 2. The resemblance to the GWR 56s is surely no coincidence, but in point of fact the conversion wasn't carried out until after the 56s had been in service a few years.




Two Ms were withdrawn in the 1930s, but the rest survived the war, three getting to British Railways and the last going in 1951.

The dark Brunswick green I've chosen to approximate (with lining missing) RR paint tends to hide detail doesn't it. I shall have to think about that.

Edited by JimC
updated graphic

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This is I think a ex-RR P class loco, not sure what the differences are to the M (frame length?), but I think they were later lumped into the same category after Swindonisation. The pic, at Cardiff in November 1923, shows a standard repaint in Swindon style of a non-Swindonised example. If the repaint was done at Caerphilly, it proves Caerphilly was obeying Swindon orders for local repairs immediately after the grouping.




Edited by Miss Prism
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Yes that's the P class (only one by that date) and a very useful looking photo when I come to it. I believe the Rhymney locos rarely went to Swindon, and that they were the best maintained fleet at the grouping. Very much in RR condition there still with pop safety valves. 

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Differences between P and M classes were that the Ps had air brakes for passenger working, converted to vacuum post-grouping, with 5' diameter driving wheels, while the Ms had 4'6" drivers and no continuous brakes.  P was to indicate Passenger and M to indicated Mineral versions of the basic Stephenson design, which became a sort of South Wales standard, the Rhymney also using it in the A class and versions being built for the Brecon & Merthyr and the Neath & Brecon.


The 56xx class may be considered as a Rhymney M or R Stephenson built out of Swindon standard components. 

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9 hours ago, The Johnster said:

The 56xx class may be considered as a Rhymney M or R Stephenson built out of Swindon standard components. 

Yes, although the 56s did have piston valves rather than silde valves. They also had an all new motion design which AIUI those that understand such things regard as being theoretically superior to the older style layout that continued to be used on six wheeled classes with inside cylinders. 

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Another big difference was the provision of vacuum brakes for passenger working.  The new inside motion was, IIRC, to cope with the cylinders which were the largest inside cylinders used on the GW, and the only inclined ones.  This is what accounts for the very large balance weights on the centre driving wheels on this lock.  One wonders if Collett considered a 5'2" wheeled version, as he has form in the matter of messing about with driving wheel diameters on otherwise similar locomotives (54xx/64xx, 61xx/81xx), but ther were plenty of Taff As about, recently built and very successful with the no.10 boiler that he designed originally for the rebuilds of this class.  It was later used on the 2251, 94xx, and 15xx.


The 56xx proved highly successful on Valleys passenger work notwithstanding the smaller driving wheels.  High speeds were not required on Valleys services, as line speeds were low; the Taff was 50mph north of Radyr, as was the whole of the Barry including the Vale of Glamorgan section, and the Rhymney was similarly restricted north of Ystrad Mynach; the Monmouthshire Valleys were hardly racing ground either. 


The 56xx, which I am old enough to remember pulling passenger trains, gave a very noticeable piston surge in the leading coaches, especially when pulling away from stations going uphill.  The GW seems not to have been inhibited by wheel size on locos intended for passenger work, with the 44xx going as small as 4'1", which proved a step too far and the small prairies were soon altered to a 4'7" specification.  The 44xx were kept as built though as they found a niche on sharply curved branches, being associated with Princetown, Much Wenlock, and Porthcawl.  This last was speed resticted to 15mph throughout and one particularly wiggly stretch to 5mph for all locos except 44xx, which were turned at Tondu halfway through their daily duties in order to even out the flange wear!

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Posted (edited)

I found a nice works photos of one of the Ms as built, and decided that, with the dimensions in RCTS, gave me just enough information to attempt a sketch of the M Class as turned out by Stephensons. The main difference is a smaller diameter boiler, which I think is just different enough to lose some of the brute strength appearance of the other 20thC RR classes. 062T-Rhymney-M-ClassAsbuilt.jpg.72268e87232909173d7eceee024b179b.jpg

But there's one little peculiarity I'd like to highlight. In this sketch the tank/cab and cab/bunker corners are both radiused, as are all 4 corners of the cab. This is as per the Stephensons' works grey photograph in wikipedia and other places as shown below. All these corners are also shown radiused on the GWR arrangement drawing for 106/47 when she was fitted with a GWR boiler. However all photographs of the class (with the later RR boilers) I have found show a 90 degree transition at bottom left of the cab door/window cutout, and between cab and tank, and a radiused transition between cab and bunker. Photographs of 106/97 with GWR boiler and bunker , however, clearly show both cab/tank and cab/bunkers with 90 degree transitions, whilst the cab window corner is hidden behind the cab shutter. So I've amended the GWR sketch to show what's in the photos rather than what is in the works drawings. RCTS doesn't record whether it was Swindon or Caerphilly factory who ignored this detail from the drawing office.

Edited by JimC
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