Inspiration

[faraway]

One of the customers I used to service, had a siding that ran the length of their building, inside. It was always interesting to watch the switching of cars on that "hidden" siding.
If it was a case of knowing how many and where, inside, here's a thought; Overhead lights, and holes through the roadbed, with a mirror under the layout, set so you could see the lights through the holes. About as simple a "detector" as I can think of, and useful if there would be "less than three, or only one", in the siding.
......and interior lighting too!

The interesting things is the close functional relationship of the lamp in a refrigerator and a Kadee coupler in the building.
When ever you have the door open is the lamp bright shining. So is the coupler always functioning perfect when you look at it. But if you close the door of the refrigerator or push the cars in the building.... it's a mystery what happens in the darkness