The operator looks down the train to make sure all the doors have closed. There is is doors closed light on the outside of the train on each car. The lady stepped out of the car and the door closed on the leash. How thick was the leash? Obviously not thick enough to cause the door closed light to extinguish. So everything looking ok, the operator closed his window and returned to his seat and the train automatically took off. BART trains can run and make all stops without an operator. Really. It was designed that way from the beginning.

When the operator looked down the train all he would have seen is a woman outside the train and all lights on. Could he see the leash? A BART consist can be up to 710 feet long.