Don’t know if the tank was full, but it had to have some gas in it to drive the cars on and off of the rail cars and the transport trucks. As for the keys, I imagine they shipped with the cars. If not, this was in the days when hot-wiring was common. All I know is my uncle caught guys in cars. Sometimes they were “rough” with bums on trains back in the day.
I can see bums sleeping in cars for comfort on the seats and with the windows down. Just kind of amuses me if they were actually riding on the train with the windows up and AC and the engine running.