So basically then, if I pitch up at the station and am refused travel without justification there would be nothing I could do?
Thats about the size of it! Clearly this is something that wouldn’t happen in practice, but that is technically possible. In just the same way as your local pub could refuse to serve you a pint, or your local newsagent could decide not to sell you a paper.
However, we will never get a definitive answer to this because it would take a refusal to travel decision to be made by a TOC and for the matter to be challenged. Despite them seemingly having the right to deny travel for no reason at all and some of them being more than eager to restrict travel, none has done so. I can only wonder why.
Most likely because that would potentially discriminate indirectly against disabled people who cannot wear a face covering. THAT could certainly be challenged under the Equalities Act.