IOTL, the government considered nationalizing the railways after the Great War before Grouping was settled upon. It appears it got far enough that standard engines were being considered, or at least Maunsell's N class mixed-traffic engines were considered for one. 100 were ordered from Woolwich Arsenal to stimulate the post-war local economy, but could it also have been a carry over from preparation for nationalization in some way?
Supposing Britain's railways are nationalized in 1923, what happens to the railways from here onwards? If Maunsell is chosen to be the CME, how will mould Britain's engine design?
Tagging
@NOMISYRRUC
I do know that nationalisation was considered, but have no idea about what form that nationalisation would have taken.
IIRC Churchill suggested nationalising the railways and running them at a loss to help the economy. However, he was also the man who also said, "Civil aviation must fly by itself!" It couldn't and subsidised foreign competition quickly forced the fledgling British airlines to stop flying.
What I want to say is that this British Railways did the best things that the Grouping companies did, but on a larger scale. However, as I subscribe to the A.B.C. theory of history it is more likely that they did the worst things that the Grouping companies did, but on a larger scale.
For steam traction we probably see what the OTL British Rail did brought forward 25 years. So a small range of standardised locomotive designs is introduced from 1926 instead of 1951. The same would happen with carriages and wagons. They would also try to reduce costs by having the locomotive, carriage and wagon works build a smaller variety of designs in the hope of achieving economies of scale.
IIRC the OTL British Railways inherited 400 classes of steam locomotive in 1948. My hope is that there would have been fewer than 50 classes by 1948 ITTL. However, that might not be of any financial benefit to British Railways. It is very likely that the Government will make the Company reduce its fares or pay the shareholders bigger dividends.
Furthermore, making steam traction cheaper reduces the financial benefits of dieselisation and electrification. That would be a bad thing because steam would linger on for a decade longer than OTL. On the other hand when dieselisation was eventually carried out it might have been more successful. There might have had time to test the prototypes properly before placing bulk orders and a reduction in the number of designs.
Note
A.B.C. = Another British Cockup