wrote

So, what is Mail Rail's ultimate fate, then?

It deserves a new use.

The Tower Subway started off with a small cable car, then became a footway,
which closed when Tower Bridge opened. It was then used for hydraulic power
mains, and water mains were also put through it. After the London Hydraulic
Power Company closed (in 1977) it was used for fibre optic cables.
http://www.subbrit.org.uk/sb-sites/s...ay/index.shtml

Peter