A better question to ask is, are you driving the train yourself or telling the train to drive itself via commands? Also, what is the exact name of the route and session in which this error is occurring, and can you provide a screenshot? (Press the "print screen" key in-game to take a screenshot. The screenshot gets saved into "<TS12-install-directory>/UserData/screenshots". The default TS12-install-directory is "C:\Program Files\N3V Games\TS12" for 32-bit systems and Windows XP, or "C:\Program Files (x86)\N3V Games\TS12" for 64-bit systems including Windows XP 64-bit.)
Since you're new, I'll also give you the same advice that works for me: Login to an administrator account and go to "C:\Program Files\N3V Games\TS12\bin". Right-click each .exe individually and select "Properties". On the Compatibility tab, check off "Run program as administrator", and hit OK. If User Account Control is enabled, follow the on-screen prompt in the affirmative to apply the changes. This should allow Trainz to run a bit more smoothly, though Trainz installed in any Program Files directory is sub-optimal. The point is, following this procedure should reduce the frequency and severity of problems with Trainz.