Another thing to check is what your PC has running in the background. Some apps are very memory-hungry. For example, I was running "Einstein At Home" on Boinc!. It was supposed to idle while Trainz was running, but wasn't, and my frame rates were very bad when running through built-up areas. I eliminated that project and Boinc! this week, and voila, Trainz is smooth as silk.
:B~)