"Bruce" wrote in message
...
On Thu, 13 Aug 2009 20:38:46 +0800, "DW downunder" noname wrote:

The internal diameter was (IIRC) 12'3" (3734mm) for cast iron and 12'6"
(3810mm) for concrete lining segments. This compares with the Yerkes'
standard of 11'8 1/4" (3562mm).


Correct.

The missing statistic is the post-Yerkes standard, which was 11' 6".


Whoa! 11'6" was the diameter of tunnels in the central section of the
Central Line. Unlike the C&SL, the trackbed wasn't broken up in the Central
on enlargement, but segments were inserted to give an eccentic profile,
large enough to accommodate stock which ran through 11'8 1/4" tunnels. As we
have found, that created a need for a raised third rail and associated
("hi-lift") collector shoe beam.

Also, post-Yerkes, most new Tube tunnels have been lined at 12'3 or 12'6 -
not sure about the JLE.

DW down under