Previously posted in another thread a few years ago, but to make the inside of your tunnels look realistic and really stand out, cut a sheet of heavy duty aluminum foil (not the flimsy standard kind) to just slightly larger (1/8") than the inside height and length of each side of your tunnel walls.
Crumple up each sheet and then uncrumple, leaving each sheet relatively flat. Spray paint each sheet lightly with flat black paint so that a little of the shiny silver still shows through. When dry, glue each sheet to the inside of the tunnel walls ( I used hot glue) and trim, as necessary.
When a train goes through the tunnel, the headlight and other marker lamps shine on the tunnel walls and the crumbling effect on the aluminum sheets creates all kinds of angles that makes the interior walls look like cut rock faces with metal impurities shining through.
