I don't know the rule numbers but the basic idea is that authority to occupy the main track is granted in several ways by rule and if its not on that list then there is no authority and no legal means for a train or engine to be on a main track (or controlled siding).

Track foreman can restrict a train from a section of track by using a Form B but they have no means to authorize a train to be on a main track.

There is an odd and unused (in my experience) exception called "Train Coordination" or something but I hesitate to even mention it. It has to do with an emergency situation when communication with the dispatcher does not exist and the track man is working on the main (but then how did the foreman get authority?).