Place it at the cut from waiting on the platform to the train pulling away, then follow it with a door knocking sound as someone tries to get back in, and a dun dun dun sound when the train is gone for good.
关于此声音
This clip is a metro door-closing buzzer lasting about ten seconds, a sharp electronic warning tone that repeats as the doors shut. It feels urgent and slightly stressful, like a last call to step back or hurry aboard before being left on the platform. It comes from the warning system used on metro trains to signal passengers that the doors are about to close.