In addition to MSTS, there is TRS2004/06. There is a lot of narrow gauge equipment available. I can't say how accurate the steam locomotive driving experience is, but the steam sounds aren't set up great. (2 chuffs per driver revolution in DCC, 1 chuff every 150 inches in cab mode) There are ways around the first, but there aren't any around the second. It is possible to get more detail in engines than it is in MSTS (MSTS has a poly limit on its engines, but Trainz doesn't). While I haven't run MSTS in a while, it seems like routes for Trainz are usually more detailed than MSTS routes. The lighting in Trainz isn't as good as MSTS, but by adjusting the light colors you can make it look a little better.
A few websites with downloads:
www.steammachine.com/prowler - for D&RGW and RGS equipment
www.steammachine.com/bdaneal - for assorted american equipment
www.trainzone.co.nz - for logging, SR&RL, and PBR equipment. The Murchison route here is superb.
There is also some narrow gauge stuff on the Auran Download Station, including several routes.
Cheers,
Ben