Nigel Callaghan wrote:
I stood on the Thames embankment yesterday, outside the old County
Hall Building, just below the steps leading down from Westminister
Bridge. I could feel the vibration of the Jubilee Line trains
passing under the river, between Westminister & Waterloo.

This seems very unlikely.
1. AFAIK the Jubilee Line tunnels run diagonally across the river from
Westminster station to Jubilee Gardens on the downstream side of County
Hall. (There was a shaft from the gardens down to the tunnels during
construction of the line.) I don't know the exact line, but if they
were straight, the tunnels would have been around 200 metres from where
you were standing. John Haines's post suggests they are somewhat closer
than that.

2. The materials and design of the Jubilee Line tunnels would almost
certainly not enable you to feel any vibration even if you were standing
directly over them.

3. With all the vibrations in the area caused by traffic on Westminster
Bridge, equipment being used during the bridge repairs, etc., it would
not be possible for the casual observer to link any vibration he felt
with a tube line or any other invisible source.

What did worry me a little, was a largish barge moored on the
downstream side of Westminster Bridge, close to its central arch
They were driving in some large steel tubes using a pile-driver. I
began to wonder just how far away from the tunnel they might be and
how deep it lies under the river bed.

I don't know the tunnel depth below the river bed. The lower
(westbound) platform at Westminster is 30 metres below ground level.

--
Richard J.
(to e-mail me, swap uk and yon in address)