--- "Chris Game" wrote:

Or one of those detector thingies they have on the back of BMWs to
detect an obstruction whilst you're backing..

No moving parts.


Why bother with a hardware solution at all? It can all be done with
software.

The onboard computer already has a list of stations that the train will be
calling at, for the scrolling electronic displays: "This train calls at..."
It also has to keep count of which stations it has already called at, so it
can update the displays each time: "The next station is..."

Just use that information to decide whether or not to open the doors at each
station. Problem solved.