Chris J Dixon wrote:

Ware springs to mind, and most termini. That sounds high-tech
enough to break down a lot, and requires fitting of kit at
stations, which are the most easily vandalised part of the
railway. I was thinking of a projecting metal arm under each
train door which prods the platform.

Considering the range of actual platform dimensions, that is
quite a large prod. Plus all the gear to ensure it is proved
retracted and locked whilst the train is moving.....

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

No moving parts.

--
Chris Game

"Hopefully the net-dwelling paranoid delusional conspiracy theorists
won't descend upon me " -- Chris Pratley, MSFT.