True, a desktop is always better, but I understand that sometimes having one isn't feasible (like in college). I have a laptop that runs Trainz 2006 without trouble on small/medium routes, yet struggles with big routes like Marias Pass or detailed ones like MoPac: Route of the Eagle. But I also bought it for the battery life of over 4 hours (12 cell battery, only surfing the internet). I can get roughly 2 hours of playing games on the High Performance power setting.
My laptop specs: HP dv6-1260se Artist Edition 2
AMD Turion X2 Ultra ZM-85 2.3 GHz dual core
4 GB DDR2 memory
500 GB 5400 RPM hard drive (a little slow, not really noticeable)
1 GB DDR3 ATI Radeon HD 4650
16.0" LED backlit screen @ 1366 x 768
All of this cost me about $1,000 dollars last summer, but I'm happy with it because of the superb battery life (I take it on the road more than I game with it...I have a desktop for gaming).
Okay, if I had to make a blind recommendation (ie, without a set price), I would have to recommend this Acer:
http://www.newegg.com/Product/Product.aspx?Item=N82E16834115839
It has a fast processor, good graphics (to play any version at full 1600 x 900, maybe even with AA/AF), plenty of memory & hard drive, and a large screen to enjoy Trainz with.