On Thu, 18 Aug 2005, Neil Williams wrote:

On Thu, 18 Aug 2005 15:27:52 +0100, Tom Anderson
wrote:

(i assume because digging wide deep tunnels was ruinously expensive).

I suspect so. A modern equivalent, the Merseyrail Loop, is very
nearly[1] full mainline loading gauge because modern tunneling
equipment made it rather easier.

[1] PEP-derived EMUs, e.g. 508s, fit, but not the slightly larger
Mk3-derived units like 455s, as I recall. The structure gauge
difference can be seen quite nicely in those 455s that have a spare
508 car inserted. Not enough to have a major impact on passenger
comfort or internal layout, but enough to be visible.

I'm afraid i don't know my PEP from my Mk3; do you mean that the tunnels
are smaller than W6A gauge? Or are these Mk3 things bigger than W6A?
Building a tunnel infinitesimally smaller than the standard loading gauge
seems like the height of madness - for a negligable saving, you throw away
the ability to every kind of passenger train, present and (near) future,
without worrying about the details of its size!

tom

--
On two occasions I have been asked [by members of Parliament], 'Pray, Mr.
Babbage, if you put into the machine wrong figures, will the right answers
come out?' I am not able rightly to apprehend the kind of confusion of
ideas that could provoke such a question. -- Charles Babbage