It's all gone quiet again on the thread, so:-
What if the 1923 sectorizations still applied with LMS, LNER, WR and SR still having their autonomy.
Hmm...
The LNER was pioneering main-line electrification, and had EM1s and EM2s under construction. They also had two EMD diesels on order (I think it was EMD). Therefore, the Woodhead electrification might well have spread to the northern ECML, which the North Eastern wanted to do in the 1920s. King's Cross is most likely where the wires would have gone next. Probably most of the LNER main lines would have been wired by the 1960s, with diesels operating freight away from the wires and some form of DMU (presumably) on local branch workings. Steam might have hung around into the 70s, with Thompson's standardisation programme basically complete, so the remaining B17s would have become B2s, the O4s would have become O1s and so on. Quite a few closures might have happened, but the Great Central would be open to this day.
The LMS had diesel-electrics under construction, 10000 and 10001. These were built by English Electric, so we can assume that a large-scale order for a similar design might well happen. As a result, we might have ended up with something resembling the Class 40, and possibly various smaller designs as well. I honestly have no idea whether we would have seen 25kV electrification. They might even have ordered Deltics for the WCML. I don't know whether Riddles or Ivatt (whoever was in charge) would have designed something resembling the 9F.
The GWR had gas turbine locos on order. It's quite possible we would have seen a mass order, but if they had not lived up to expectations, perhaps we would have seen diesel-hydraulics begin to appear. There were already plenty of diesel railcars, so I imagine that DMUs would not be that far a step. There's also a possibility that we would have seen the Hawksworth Pacific, probably the
Cathedral class, as the GWR's last major steam design.
The Southern had steam, diesel
and electric options. Bulleid's first five
Leader-class engines were under construction, and 25 more were on order. They might not have done very well, but I reckon that there's a chance that they might have been made to work. They also had three diesel-electric locos on order, plus the electrification programme, which had produced three electric locomotives and loads of EMUs. I imagine that the Kent Coast and Bournemouth electrification schemes would have gone ahead much as they did, and that the third rail would eventually reach Weymouth and Plymouth. Competition with the GWR would help keep at least the Plymouth route open.
Would we have seen all four companies co-operate in the 1970s to build the APT and HST? That's a complete unknown. It's quite possible that we might today see 125 mph "EM11" electrics operating out of King's Cross and Marylebone, with "EM10s" out of Liverpool Street. Perhaps there would be tilting EMUs out of St Pancras and Euston, 125 mph gas turbine sets out of Paddington and several classes of EMU out of Waterloo and Victoria. Freight might be in the hands of various things, with EMD locomotives ordered from America being likely. Also, the various types of local multiple unit would be widespread, the Leyland "Pacer" having been abandoned in favour of the Metro-Cammell "Sprinter". These would be on the wane by now, in favour of the Bombardier "Turbostar". Off-the-shelf orders from private trainbuilders would result in something similar to today's reality.
Presumably, all four would be dependent on subsidy for passenger services, otherwise we would see something line the Serpall Option A network, plus a few freight lines. Heaven knows whether we would have got a Channel Tunnel, but I imagine that we would, and it would have 1,500V d.c. electrification, to match that pioneered by the LNER.