If a train operator transmits a 12-9 which means a person is under the train, the emergency crews have to rip open the gangway in order to retrieve the person, depending on where the person under car is.
If one wheel comes off the track, then the second one comes off as well. If the entire truck comes off the track, then both axels that the truck holds are derailed.
NYCTA F subway line. Only 37 passengers were on the train with no injuries. From where the subway car landed when it came off the tracks, a few feet more and it would have come off the roadbed and onto the street.