High tech solutions are not always appropriate.

A simple lightly sprung door can operate a mechanical microswitch.

1. easy to clean and maintain by unskilled operators.
2. can work in the dirty conditions.
3. microswitch is cheap and reliable.

Photocells, lasers, magnets, are more complex and susceptible to dirt, dust, iron particles ...

Some years ago, some friends from our computer club were drinking a beer at an outside cafe in Amsterdam (I think). As a tram approached, we speculated as to which track it would take.

We suggested computer controlled points, CCTV, hand of God, ...

In fact what happened was that the tram driver approached slowly. Examined the state of the points. Took an large crowbar from the tram. Set the points to his chosen route. Climbed back into the tram. Continued his journey.

David.