These are HO but the same idea works for any scale. Construction is plywood with a thin coat of plaster sanded smooth to cover the grain. I patterned them from actual SP concrete tunnel portals built 1910-1920's. I made these wider than scale to accommodate trains on curves. It's a good basic design that looks authentic.
Last photo is tunnel 21 near Oakridge Oregon. I've seen other versions with vertical wing walls, which are somewhat easier to build.
