Here is a 2d parametric rail (metric).
Save both files in a library folder, reload the folder, and drag the rail into your drawing.
Stretch or edit nodes to change lengths.
change other parameters via the selection info pallete
Your drawing should have 2 layers - whipeout and rails (optional, otherwise the contents of the rail will go to layer 0)
on the whipeout layer is a solid white hatch which should hide anything behind the rails - so that you dont have to trim.
This relies on "convert foreground colours.." in the backgroud colours palette on the to be turned off.
But unfortunately I cant get the hatch to work with the cross style rails.