A buttkicker is the last thing I would add to a Trainz PC.
There are just too many other ways to put that money to better use. Of course if everything in your computer is top of the line and you have the money then go for it.
I've played this game on my home theater setup (42" screen 5.1 audio with a 200 watt subwoofer) and on my little computer room setup with a stereo amp and a pair of infinity book shelf speakers. Bigger is better, but there isn't anything special sound-wise about this game other then the new doppler.