Switch timer for display layout

The easy part is a timer module inside that sets a relay for 1-2 minutes (or whatever you choose) when triggered.  That's a $5-10 solution.

I'd think the hard part is how someone on the outside triggers it without messing with wires through a wall or window.  To that end I think a wireless-doorbell might be worth looking into.  The button-side is battery-operated and sends a wireless signal to a receiver inside which would trigger the timer. 

Stan, could some type of motion sensor possibly be adapted to work here? As in maybe 'Place hand here to run train', hand would go over the sensor?  Not sure they have one that is that narrow of a range?  I suppose most of them would probably go off anytime someone went by, whether they wanted to see trains or not?  (Just a thought that you probably already considered.) The door bell button would be much more selective.

What would be interesting about a motion sensor is if it could be mounted inside the window looking outward.  That way nothing is outside.  Let's see if the OP thinks having nothing mounted externally is important.

In which case, if one is a DIY'er I can imagine an obstacle detector module that shoots an Infrared beam outward.  If a hand reflects the beam, the sensor triggers the timer.

Or if messing with modules and such is too hands-on, something like a Lionel 153IR could be mounted up against the window.  These can have a range of up to a foot or so which I'd think would trigger reflecting off a hand.  Only problem being if we're talking a cold winter day during the holidays what about folks with black gloves?  The 153IR can be finicky detecting dark-colored engines or rolling stock!

I did this type of thing for a display layout many years ago. Just pick up a motion activated light at the home improvement center. With a little rewiring, set it up so that your power supply plugs into the output side of the sensor. Most of them have a delay adjustment so you can set how long they stay on. This can all be done behind glass.

Needless to say, there are a variety of motion sensing technologies.  A common one is PIR recognizable by the multi-faceted sensor like on the right side of this which you might find at a home-center:

These look for variations in body heat thru the detection of Infrared (IR) energy.  Problem is most windows these days are thermally insulated (e.g., double-pane).  This makes detection a your-mileage-may-vary; hence some experimentation required.  And if a cold winter day with folks bundled up, sensing body heat becomes that much more difficult.

There are other motion detecting technologies like microwave that can transmit/receive through glass.

Thanks to you all for your suggestions. I like the idea of trying a motion sensor from the inside. Now I’ve got some great ideas and about 10 months to get this done.... we all know it will be here in a blink of an eye. Thanks again and Happy New Year!!