As well as Box tunnel...
I worked in a top secret Naval Base in Plymouth (o.k., you've forced it out of me. It was Devonport) some years ago and there is an old railway tunnel there, used to be part of the dockyard railway, which is disused and sealed off. There was,, however, a rumour amongst the train spotting community that it was used to to store some old steam locos, as part of some sort of strategic reserve.
The MOD Plod used to regularly arrest members of this lot for breaking into the dockyard with intent to write down train numbers.