I know an old chap who ran a large scrap company - they won a contract to clear a landmine factory on the Rainham(?) marshes, in the early '70s I think, which had a very large internal railway system spread over several miles complete with two fireless steam locos.
The rail track was taken up last. He got a call from his foreman saying "I hope you didn't pay too much for this railway line - it's cr*p metal, the crane magnet won't pick it up".
He hot-footed it down there with a file and magnet, and promptly discovered the entire rail system had been constructed with gunmetal rails to prevent sparks, but nobody had remembered or noticed that. He was quite pleased with that discovery! :-)