The easiest way to model a railroad roadbed would be to use styrofoam and cut it to shape with the bevels in place.. Then lay your ties, then the rails, then lay down the ballast...
Your scale is going to be "l-Scale", which is approx. 1/32nd, but that's close enough for government work.. Here's a rough sketch of the cross-section..
I'm working a diorama right now that has RR tracks in this scale, so any questions about dimensions & materials, feel free to ask...
Oh yeah.. Kitty litter works for ballast, but not in this scale.. You'll likely want to use mine tallus instead..