I really can’t buy the idea of not being allowed to SRM through Allied controlled tt.
To all intents and purposes, each alliance had a single rail network (at least in connected regions), and indiscriminately transported units around it regardless of nationality.
Even in the case of German and Russian areas (which had different gauges), new lines would be laid down, and rolling stock converted to join newly won areas up to the networks. Why not allow Germany to rail an army from Berlin to Baghdad; isn’t it up to the Allies to cut the link by capturing a connecting tt?
Another thought: all SRM must involve your capital. That is, the army to be railed must either start or end the move in the capital, but can travel to or from any other connected friendly tt. That way the capital acts as a natural hub of your movements, but you cannot simply warp an army from one front to another; in effect it must return to the capital for redeployment. Think of it recharging its batteries and refitting before being sent to a new hellhole.
The problem is for the UK and US; could they use Paris as a transport hub for units on the continent?
Perhaps a power can use any friendly capital as a SRM hub?