This is a great picture, you can click on it for more detail. The article has two estimates to turn it into a bicycle tunnel: 3 or 30 million pounds. I would go for the higher, since I think it is almost as much as to get old musky over and dig a new one.
The bricks are weathered and the tunnel leaks. This is okay for an old steam train, but falling bricks make for a bad day, for cyclists. This tunnel has to be relined with a waterproof liner, steel mesh and shotcrete. Then steel linings for the ventilation shafts, fans, pumps etc. Now that I think of it, even the high estimate is low, since we must have power and wifi. :)