If we assume that RRs represent the evolution of modern transport systems, why not have it represent both carrying capacity and infrastructure. This capacity could expand with the track system through new techs, new resources, new buildings and shields.
For example, when you first discover Steam power and link up coal you have the ability to build "X" RR tiles where "X" is equal to the distance from your capital to the furthest city on the continent. The tracks could be built anywhere, but the number of tiles is determined by that distance. Furthermore, the number of units you could move by RR in a turn would be equal to, say, half the number of cities you control. As your empire grows, rail capacity would grow naturally.
Production endeavors could change the calculation of these limitations. Players could build RR stations that would add to unit/turn capacity or they could build "tracks" in cities; these tracks would be built in blocks of 10 or 20 squares and would add to the number of RR tiles you could have workers complete. Once 20 tracks are built in city production, workers could lay 1 square per turn per worker on the map.
New techs (refining, motorized transport etc.) or new resources would automatically increase capacity multipliers and reduce cost of new track production.
I would add a maintenance cost per track tile that you had the option to pay and if neglected too long, capacity would drop and track could disappear (like pollution effects).
Then as a warmonger, I could bomb individual track tiles to limit delivery destination options for the AI or I could attack train stations in cities to reduce capacity. As an empire collapses, rail capacity would drop also.
With such a scenario, I think that rail growth would be slowed and better reflect changes over time and we wouldn't have quite the unbalancing effects of unlimited units over unlimited space. As a human player I think this would add to the game. I don't know how or if the AI could handle it.
