What's the common distance between rails in source maps?

So I am making new rails in 3ds max for my map. And was wondering, what’s the usual/common distance between 2 rails in source maps? By saying common I mean so that PHX wheels, or HL2 train model fits on them? I know I could make 1 rail and then just place it beside another, but I will do it other way. SO anyone knows it? Specify it in hammer units please.