I'm building the current D&J Railroad with drywall backdrop material. I had used the brown masonite board on the previous layout, but it seemed to soak up the paint I applied for sky blue and the brown would show through.
The masonite can be used for the facia material as it is sturdy enough to be self supporting and holds toggle switches, push buttons and indicator lights quite well, and its very affordable compared to plexiglass.
I suppose you could use a combination of masonite with inset plexiglass panels.