The rain particles get converted to splash particles when they collide with opaque surfaces.

Because the particle system uses the depth buffer to detect collision, and since the depth buffer doesn't have information about transparent surfaces, rain particles simply fall through stuff like glass. (Or we can say that New Alexandria's glass floors are built from Awesome Future Glass, safer than normal glass because it doesn't get wet in rain.)

Rain does collide with dynamic objects, although I'd have to take a closer look to see what exactly it does. I'd expect that there wouldn't be too much per-surface customization, as the entire gist of Reach's particle system is to be efficient by catering to a simple general case. Maybe I'll check that out later.