I think nobody knows for sure what the values mean.
Well, actually
this guy sounds like he knows but my head asploded trying to read his text. It's been a long day.
The point is, these are standard EAX parameters, not specific to Railworks, so you can search the web for things like "Occlusion LF ratio" for information about them. Keep us posted.
Being lazy, I'd say find the passenger cars that have an occlusion file, and try them in the game until you find one you like, then copy this file for your purposes.
Also obviously it only works if you are using EAX in the game.