This happened the other day in Madrid. A 41-year old guy stumbles and falls on the track of the Madrid Metro, in the station of Puerta del Angel, shortly before a train is due to enter. The passengers that are on the opposite platform see it happen, and start shouting and waving frantically to try and alert the driver of the train, so that he can stop.
A 30-year old off-duty rookie police officer (fresh from the academy; had taken his oath of office just 2 months earlier) who was out shopping with this girlfriend went down on the tracks and pulled the 41-year old to safety literally seconds before the train arrives. You can actually see some of the people on the other platform turn away, not wanting to see what they feared might happen.
It was all caught by a CCTV camera. You can see the whole thing unfold here: