ok, most people don't like infinite move rails because it leads to the strateguy of pooling a huge rapid response force in the centre of your territory, able to smash anything on any border at a moment's notice. While realistic, this is also not fun, as it effectively removes the need for any forward planning beyond the need to have troops around in the first place.
otoh, Having finite-move rails (aka very fast roads) doesn't make sense, as there is no good reason why a cavalry unit should move faster than riflemen on rails.
otth, Slowly moving entire armies across the entire map is not fun.
So...
Take rails off the map entirely. Have "roads" and "highways" as tile improvements, and "rail depot" as a city improvement. Assume an abstracted rail line exists between any pair of cities in the same nation that have rail depots. A rail depot can do a civ2 style airlift on any unit. This action uses the unit's entire move, and costs a small gold fee. This allows for quick troop movements, avoids the realism problem of the enemy using your rails, makes all units move on rail at the same speed, and makes an opportunity cost (the rail-lift fee) for usng the central reserve force strategy.
otoh, Having finite-move rails (aka very fast roads) doesn't make sense, as there is no good reason why a cavalry unit should move faster than riflemen on rails.
otth, Slowly moving entire armies across the entire map is not fun.
So...
Take rails off the map entirely. Have "roads" and "highways" as tile improvements, and "rail depot" as a city improvement. Assume an abstracted rail line exists between any pair of cities in the same nation that have rail depots. A rail depot can do a civ2 style airlift on any unit. This action uses the unit's entire move, and costs a small gold fee. This allows for quick troop movements, avoids the realism problem of the enemy using your rails, makes all units move on rail at the same speed, and makes an opportunity cost (the rail-lift fee) for usng the central reserve force strategy.