Even though I haven't decided on a laptop yet, I've been looking at some that might be pretty good for gaming:
$1,299.99 ASUS N70SV-X1 - Super fast Intel Core 2 Duo T9550 2.66 GHz and half-decent nVidia GeForce GT 130M (the exact same as the 9600M GT chip, but with 1 GB of memory) 1600 x 900 in a 17.3" screen. Also has Blu-Ray and twice the hard drive space as the one below. A little better value-for-the-money.
$1349.99 MSI GT725-074US - Excellent Mobility 4850 graphics & turbo button to overclock the Intel Core 2 Duo P8600 processor from 2.4 GHz to 2.7-2.8 GHz when plugged into power outlet. 1680 x 1050 in a 17.0" screen size. Better performing than the one above when overclocked.
Both of these I would recommend for playing Trainz. There are definitely faster laptops out there, but these are some of the cheapest gaming laptops that I've seen in a while.
Hope this helps!
Kyle