There have been many debates about whether you should be able to use roads and railroads in enemy territory. Both sides have good points to make, so I think a compromise is in order. Rather than have a blanket yes/no to using enemy infrastructure, there should be a "secure" action. This simply means that the unit, presumably on enemy road or railroad, secures the area and enables that road or railroad for use by friendly forces. "Secure" is an action that uses up that unit's turn. Once a tile has been secured, it remains so as long as it is occupied by friendly forces. In other words, it does not need to be re-secured every turn if a friendly unit is on that tile at the end of the turn.
This would make paratroopers more useful (and historically accurate). You would launch your paratroopers at the start of your war to secure roads, railroads, forts, etc. ahead of your main invasion force (i.e., on tiles more than 1 turn distant). They would secure the roads and then buckle in for a rough ride as the enemy tried to take those tiles back. If they survive, then your following units have a clear path the next turn. If they don't, well, better luck next time ;-). You could of course secure terrain with other units, such as 3-move cavalry when you're deploying 2-move tanks. That might be too powerful, though, so "secure" could be a special ability that only some units have.
This would make paratroopers more useful (and historically accurate). You would launch your paratroopers at the start of your war to secure roads, railroads, forts, etc. ahead of your main invasion force (i.e., on tiles more than 1 turn distant). They would secure the roads and then buckle in for a rough ride as the enemy tried to take those tiles back. If they survive, then your following units have a clear path the next turn. If they don't, well, better luck next time ;-). You could of course secure terrain with other units, such as 3-move cavalry when you're deploying 2-move tanks. That might be too powerful, though, so "secure" could be a special ability that only some units have.