Title says it all, really.
When using Railcraft, where can sulfur be found?
I've found a few hints that may point you in the right direction. From the patch notes:
From a list of coding thanks for another mod:
These make me think that you may find sulfur around pools of lava on the surface.
After some nuke-based prospecting and using the hints provided by SaintWacko, I've managed to find the answer:
Sulfur spawns near underground lava lakes in ExteremeHills biomes.
Plentiful when present, but the conditions can be hard to come by.