A short Rosetta Stone:

MSTS = Microsoft Train Simulator
Mark One = version 1, which so far is the only one to make it to market. Microsoft has now started, made interesting noises and marketing leaks about, and killed, a followon version twice.
ACES = the simulation group that was developing the second version 2 of MSTS, as well as the next Flight Simulator. They were shut down in the great Microsoft layoffs last week.

MSTS 1 works, and if you're lucky and have a Really Good Computer works pretty well and (with some tweaks) realistically, and because it was widely distributed has a lot of free and for-pay support in terms of add-on utilities, routes, and trains. Many route developers have gone to considerable trouble to make realistic models of their territories, some perhaps less so, and as in all such situations at least one was indistinguishable from a crook. The model development process is not for the faint of heart - a real programming project - but the results can be pretty good. Not as good as observing and riding the real thing, of course.