There is not exactly an interlock that prevents the train from moving. (Such a thing could lead to a train being stuck if it malfunctioned with a false door open indication. I think they get a lot of false indications.) The engineer, however, can see on his board an indication of whether the doors are all reporting closed. The indicators don't prevent the train from starting.

Unfortunately, those Capitol trainsets were built on the cheap, and their doors have been notoriously balky.