You are also discounting the possibility that, should we rejoin the EU, we carve out an exception for GBR and don't have to obey the same regulations.
There is certainly an argument to go that way, as given that our railway is a bit disconnected from those of the rest of Europe there isn't the practical need for through running and the likes. We would presumably need to accept something reciprocal for that, e.g. that UK companies may not bid for European rail contracts, but again I don't think that's disastrous. We could probably sensibly agree that the bit of the railway that does overlap with theirs, HS1, could come under EU rules as it is more like the European railway extending into the UK than the other way round.