I'm running an older Hercules (Guillimot) Fortissimo III - it's in a standard PCI slot.
It seems to work well with lots of applications (including Trainz, Morrowwind and Oblivion).
I would suggest you look for something similar - the latest sound blasters are getting good reviews, but I don't think you need that level of sophistication/cost for Trainz....
(the Fortissimo is capable of delivering 7.1 speaker system - though you'd really need a decent amplifier for top quality surround sound).
Make sure that the drivers are available for your OS before you buy! - may also be worth checking that it's sound blaster compatible
You should also check the power requirements!
Since most of the cards have their own processor, you shouldn't need a super fast bus - the instructions are sent to the sound processor - not all the data. That's the big saving on bus loading.
Cheers,
Colin