On 26 May, 12:12, David Cantrell wrote:
On Tue, May 25, 2010 at 04:51:32PM -0700, lonelytraveller wrote:
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?

The PO railway is such a small gauge (9' diameter tubes) that to widen
them would require almost as much work as boring new tunnels.

Why? The main difficulty with tunnel boring is the lack of a line of
weakness to follow, and the lack of anywhere for spoil to spill into.
The PO railway is a line of weakness already made - a 9' diameter line
of weakness in fact - and a 9' diameter tube for the spoil to ease
itself into. There are even tracks to help remove the released spoil
by means of rail. Tunnel widening is a lot easier than tunnel boring.

Also the stations are a lot shallower than the bulk of the tunnels, with
1:20 inclines either side which would be ... inconvenient.

Only if you expand the bore from the same point all the way through.
If you expand it downwards at the stations, and upwards away from
them, the incline is significantly reduced.