Hi dd - I am not a techie ...... but my 6400 had a 256mb ATI Mobility Radeon X1400 video card, 1GB memory, and a 1.83GHz/667MHz processor. These factors (I believe) make the 6400 better equipped (than 610 / 620) to handle the demands of Trainz.
Hopefully some of our more learned Trainz members will throw some comments into this thread.
Stay tuned!
Colin.