"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.