Just FYI, the OS itself has nothing to do with it; some tests even show good old XP as having a bit of an edge over newer operating systems in various categories, including DX9 gaming, like Trainz. You also should be aware that Trainz, at its present stage, only uses 2 cores and, while it theoretically can use up to 4Gb of memory, I have yet to see or hear about it using anywhere near that. And if you are thinking of a top of the line SLi or X-Fire graphics card, you're in for a big letdown - Trainz does not take advantage of multiple GPUs.
I can certainly see the argument in favor of building a reasonably future-proof machine. But train sims tend to be small-market products and thus tend not to be bleeding-edge. Thus, Trainz is going to take very limited advantage of the power modern hardware has to offer.