Peter Able wrote on 12 Mar 2018 at 19:27 ...
wrote in message news
I took my first ride on the bakerloo line for years this morning and I
noticed
that southbound from charing X to waterloo the tunnel lining suddenly
changes
into rather old looking concrete for a fair distance. Is this due to WW2
bombing or something else? The iron rings resume just before waterloo.


I believe that the sections of the Bakerloo Thames tunnels in gravel were
lined in the 1920s with a steel liner, the gap inbetween the liner and the
original cast iron segments being filled with concrete. Later on (1940s),
when the track base was being rebuilt (in concrete) the steel liner was
removed. The 1920s work was meant to stiffen and improve waterproofing in
the tunnel. I guess that it was thought that the enhanced concrete on its
own could do the job.

Further work was carried out in the 1990s, though that was mostly to prevent
undermining of the tunnel (?)

The WW2 under-Thames casualty was the direct hit on an unused bit of the
Northern line.

There was also an incident in 2012 when a Bakerloo train "hit a tunnel", which I think meant that some extremity on the train hit a piece of quick-dried cement grout that had been applied in overnight maintenance, but had oozed out before it had dried.
See https://www.telegraph.co.uk/news/ukn...asualties.html
--
Richard J.
(to email me, swap 'uk' and 'yon' in address)