I agree with Blackwatch, adding an extra internal drive is probably the easiest if you have a lot of stuff you want to keep. If you can't patch you are probably soon going to have problems running Trainz.
I assume you have done the obvious such as deleting temporary files, use something like Ccleaner and emptying the recycle bin?
Usual offenders regarding disk space are videos, mp3's and image files, which can take up a lot of space, get rid of anything you don't want or burn them to dvd.
Next thing to look at is installed programs you have never used and their setup programs if you have kept them, if you don't use a program uninstall it.