In message
,
lonelytraveller writes

The post office railway is disused.

Why didn't they just widen the tunnels and reuse them for Crossrail,
instead of digging hugely expensive new ones?

It's not deep enough - the Post Office Railway is on average 21m below
the surface, although the stations are virtually at basement level (and
so the tunnels also have steep 20% gradients either side of stations).
Crossrail in the central area is up to 36m below the surface. Also, the
Post Office Railway doesn't have a straight enough alignment - it runs
north of Oxford Street, curving up to Wimpole Street and then coming
back south before the big loop up to Mount Pleasant.

It is a shame, though, that it hasn't been put to some good use since
its closure.
--
Paul Terry