In message , Annabel Smyth
writes
Because so much of the network was up the spout today, I ended up
changing at Waterloo to catch a northbound Bakerloo Line service to
Charing Cross.

Is it my imagination, or are the two tunnels one on top of the other at
Waterloo?

Yes they are.

And where else on the network does this apply?

There are several on the centre portions of the Central Line.
--
Roland Perry