I was hunting around last night. In short:
OTTD - years of players fettling to make it better, so good trainset fun but weak economy
Railway Tycoons
- #2 is good but lacks some of the advanced track laying features
- #3 has very advanced economy. Maybe too advanced (there's a fix to get it working in Fista/7)
- Railroads = pretty, kids game
There's also a bunch of indie stuff knocking around, many of which are rehashes of OTTD. One that looks quite good and is out (graphics notwithstanding) is
Simutrans.
I'm going to give it a try.