"John Rowland" wrote in message
...
The picture in page 2 of
http://www.murphygroup.co.uk/pdf/Tunnelling/Elstree.pdf seems to show that
there is (or was) a single track railway in the new National Grid tunnel
from Elstree to St Johns Wood, underneath the A5. Despite having almost
half
the cross-sectional area of a Jubilee Line Extension tunnel, the whole 20
kilometres of it seems to have been built for the cost of 200 metres of
double-track JLE! (An unfair comparison for several reasons...)

The document refers to a monorail inspection system - a bit different from a
proper railway.
Cheerz,
Baz