Flashman,
I had similar fears of how the game might be distorted, but your explanation of super stacks gives a concrete demonstration of what I was afraid of. I will try a system similar to what you are suggesting.
1. Allow rail movement through allied terrritories.
2. No rail movement through contested territories. However, I think I’ll allow movement into and out of contested territories, provided they were already contested - and the player had at least one unit present - at the beginning of his turn. The logic being that if they have been fighting there since last turn, they’ve had time to change the gauge of the tracks on their own side of the trenches.
3. Any unit that moved or engaged in combat earlier in the player’s turn is ineligible for rail movement.
4. With the restrictions in 2 and 3 above, I think I’ll allow any number of eligible units from as many territories as the player desires to use rail movement.
5. I’m still up in the air on allowing newly mobilized units to rail move. I’ll try it both ways.
Edit: It just occurred to me that I didn’t cover railing into territories the player invaded this turn, but captured them outright instead of just contesting them. I guess you could say they haven’t yet had time to change the gauge on the tracks. On the other hand, if they overcame enemy resistance that quickly, maybe they did. It would also reward players for invading with enough force to win in one round. If you allow this, I guess you also have to allow rail into minor powers that have just been mobilized to your side (like Russia moving one unit into Rumania at the beginning of its turn, and then reinforcing by rail).