I don't like the unlimited railroads. Many of you said that in RL railroads aren't limited. THAT'S WRONG !!! They are limited to the accord of the gouvernment ! I really think it should limited to connect cities. Roads shouldn't have this limit, because it makes the size of the city.
I think railroads should have a different rolle in Civ4. It should only be used for making transports faster. With this only carateristic, a player will naturaly limit its railroads, so it should basically not been limited. Railroads in some tiles would simply be lost time, lost use of a worker.
But since railroads gives bonus of food and shields in Civ3, that bonus should be replaced. By the way, I don't see why a railroad make a field produce more food or highers production (if anyone has an answer ...). In Civ2 CTP (I know I refer a lot to this game, but it had a lot of very good ideas), there where 3 levels for farms and mines, which came with discovering certain techs. This should be taken back, but shall be done by worker in getting to the tile you want to update. But I really think that the "bonus for railroads" concept shall be taken away.
Railroads shouldn't give unlimited travel. Much like roads do by 3, it should divide by 6 the cost of travel for a unit. And also like in CTP, there should be a 3rd level of roads (Supraconductor), which would divide travel cost by 12.
A limitation in numbers (6 was the example) isn't good also. Take the example of a city that should be conected to 8 cities. That take 10 tiles to be railroaded. And you can argue with the ways you are using or the short ways you want to creat (because it doesn't give unlimited travel). So you can't limit railroads in numbers, but you can do in facts (beacause more would meen a lost of time and ressources).
The idea of having tu build a train in order to travel and train stops is completely dumb (sorry for those who had it). But I can recommand you a very good Train game : Railroad Tycoon II. REMINDER : CIVILIZATION IS A GLOBAL GAME !!!