I have seen many streetcars out of service for not only 512, but 510, 511 and 509 going to/from routes with no one on them as the driver is not allowing it. I have been on a few 512 on 511 where the driver allow you to stay on or get on at various stops as well off, including Bathurst Loop going north. Been on a few going up to St Clair.
It depends on the driver and their schedule. If drivers using King going in/out of service, picking up and dropping riders off slow them down to the point extra overtime is needed or late going into service.