- if the train is within the station limits, that lever can be active
- in case of fire the lever should alert the conductor. And the conductor should quickly make the best correct response. Often the passengers don't know that a station could be 20 seconds away and could clear out the train faster. The dispatch could clear a path for the train to speed to a better location than stuck in a tunnel with potentially nearly no place to evacuate to. Do you evacuate to ongoing train traffic? hell no! We have fire detectors, we could install them in every cart.
Modern subways have interlocks preventing motion while the doors are open, and preventing the doors from opening while the train is in motion.
Yeah, until the drivers decide that the doors are so unreliable that it's faster to override the interlocks and hope nobody's caught in the doors (which will eventually reset themselves and close properly).
On Muni's older Breda trams the interlock had a little toggle switch on the circuit breaker panel (conveniently located in the cab) you could use to override it. On the newer ones, it looks like the doors didn't detect a "pinch", the interlock decided the doors were closed enough, and the driver couldn't be bothered to check the video cameras.
Or if the doors won’t close and the train starts to move.
Or the classic: there is a fire in the train and waiting for the next station is not an option.