"peter" wrote in message

"Nigel Pendse" wrote in message
...
"CJC" wrote in message
om

The tunnels from Hounslow West to Heathrow were built using the
cut-and-cover method that was used on the district/met lines when
they were built, unusually.

There is some cut and cover tunnel, but much of the Heathrow
extension is deep-bored, including all of the line under the airport
itself. Cut-and-cover would hardly have been an option for a line
that was tunneled under an active and very busy runway, a number of
taxiways, probably some hangars and (presumably) Terminal 3.

Just because it was deep-bored doesn't mean it has to be small bore.
In this city there are deep level main line rail tunnels (for double
deck stock) as well as deep-bored road tollway tunnels (3 lanes each
way).

Agreed, but that wasn't the point I was making. The fact is that the
Heathrow LU tunnels are Tube, not sub-surface sized, so the District Line is
not an option now. Presumably in the 1970s it was a lot cheaper to build
small diameter than large diameter tunnels, which may have been one of the
reasons why there were built that way (quite apart from the other advantages
of the Piccadilly over the District line).

Of course, the Heathrow Express now has larger diameter deep bored tunnels
under both acyive runways, taxiways, etc.