d-27 disappearing layout

You could hide the handcar in the mountain and have it come out and run, then hide again.

A contactor or isolated rail to trigger a timed relay to control power. You'd have to work out the track length, speed(voltage) and timer duration for it to stop in the tunnel when finished.

Here's the D-27 in operation in case you haven't seen it. This is an original.

On my Modern Version of Lionel's Disappearing Train Layout I used a magnetic latch relay for two MTH Caterpillar sets.  One set goes into tunnel and other set comes out of tunnel.  Kids figure out there are two different trains before adults.

See Electrical Control and Wiring Information for relay control.