I am still not sure that those regs quoted by DaveL apply to railways that were never part of the "British Railways Board", as reading through quickly it seems to refer to BRB several times.
So, I would doubt they apply to SMR or F&WHRs although you might have the bizarre situation where it might apply between Caernarfon and Dinas.
I have often thought it surprising that the FR is not fenced along the Cob!
Still :offtopic: :oops: 😞