Didn't read the WHOLE thread, so please forgive me if I'm repeating what's already been said!
I have three suggestions which could effectively retain the infinite RR movement, but at the same time making it much less powerful and, better still, eliminating RR sprawl!
1) Maintainance costs for tile improvements: The more tile improvements you have, the more they will cost per turn! Also, make the costs variable, so that RR's are more expensive than roads. This way, I think you'll see RR sprawl vanish in an instant.
2) Base the commerce/food/production bonus of railroads NOT on the number of railroaded squares in your city radius, but on the total number of cities which are connected, by rail, to your trade network. Also grant a lesser bonus for road connections! This way, it building 5 RR's from a city will gain you no greater benefit than just building ONE! Again, this will reduce the tendancy for sprawl!
3) Make RR's only give unlimited movement IF you move onto it from a city! In all other circumstances, you'll simply get a 1/4 or 1/5 movement benefit (Assumes the 'underlying' road network still exists, but in a much more advanced form!) In addition, every city that you pass through, as you move to your final destination, deducts 1 from your movement allowance!
A possible 3a is to make it impossible to attack from a RR square or, if you can, that you lose all of your movement allowance straight afterwards. Plus, your defence strength will be reduced to reflect the fact that you're travelling in rail cars!
I think that these three alterations would make the placement, and use, of RR's much more strategic in nature, and eliminate sprawl FOREVER!!!
Yours,
Aussie_Lurker.