I'm lucky enough to have all (and I mean ALL) the BUR engineer reports from when the tunnel was dug, currently in a folder on my bookcase. I have studied these at great length and there is absolutely no mention whatsoever of such a secondary bore being tunnelled when the main bore was tunnelled. I would also think it highly unlikely that during the lifetime of the railway being operational such a massive undertaking was taken in a working tunnel.
There are also no overhead air vents as the shafts were infilled, though no.4 shaft (around 400 yards from the south portal) still continues to pour water at great rates and it's likely the wall around this section occasionally had to be rebuilt & strengthened. There are in fact several sections of the tunnel lining that have been renewed at some stage.
Border Collie says he saw a set of points and a signal inside the tunnel yet the Shankend Bobby, seemingly knowing a lot about the "secret", said these had been removed.
Like Steele Road, I too have walked through the tunnel, on many occasions, and have also videoed inside the tunnel aided by hi-intensity lamps. Not wishing to be a sceptic, but I have my doubts - it sounds like this is a classic tale of someone being at the receiving end of a practical joke - a good one, but a joke nonetheless, I'm sorry to say.