I like the concept of rail movement in AA. With that said I think there should be some limits. I wouldn’t want you to be able to move your entire German army (say 15 inf/10 tanks) from the Russian front to the French coast 6-7 tt away or vice versa. There would need to be some internal mechanism worked out for limits. As Flash said all ground movement would need to be modified (see note below).
Note: The above would make all ground units 1 move units (in combat). Your 2 move units could now be used to blitz into a 2nd battle, or retreat from the front lines. Tanks could still blitz through un-occupied enemy tt using its 2nd move (could toss in they only get their 2nd movement if they kill the enemy in one round of battle).
Expanding on mavericks idea:
Allow all 2 IPC tt (or more) to have limited rail movement in noncombat (would not need card board chit). This is a good place to start as 2 IPC shows some infrastructure as it is also needed to build an IC. Any ground unit starting in such a tt (2 IPC or better) gives all your (or your allies) ground units there 2 moves in non combat only. If your ground units start at an IC then they would get +1 for movement (3) still only in non combat. You could extend the +1 to any AB or NB assuming there would be better transportation near them as well. You could also allow for SBR to damage/limit rail capacity, even on tt w/o a facility. Your IC, AB, and NB would double as rail stations (it is assumed they would have one any way) so again you would not need a new card board chit IMO. This would also self limit places like Africa/China that didn’t have much as far as rail.
My group has allowed limited rail movement for AA50 capping the # of one move units you can rail through each tt based on its IPC values, but AA40 will have many more tt giving to many alternate routes. It think it would be a nightmare to track. The above would be a much easier system.