Jim Champ's "Introduction to Great Western Locomotive Development"

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 October 18, 2022 by Miss Prism

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. 

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. 

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. 

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!

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. 

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 May 6, 2022 by JimC