"Heliomass" wrote in message
...
I have a question that's been bugging me for some time. When you're
travelling northbound on the city branch of the Northern line, as you
approach Angel, the train appears to pass diagonally across an enlarged
section of tunnel (ie, an open space suddenly appears on one side and as
the
wall moves closer on approach to the station, the wall on the other side
of
the train seems to move away). Upon leaving the station, the same thing
happens in reverse. Can anyone explain this?

A new northbound platform was built, with short lengths of tunnel either
side joining the new station tunnel to the original northbound tunnel.
Because of the need to keep the service running whilst the diversion work
was done, the junction tunnels were built outside the existing tunnel, which
was later demolished and, ultimately, the track diverted.

A similar diversion was done at London Bridge, where a new southbound
platform was constructed, though in this case the northernmost junction was
done differently due to the Thames being above (which also meant that the
line had to be closed for several months). Here, part of the old tunnel was
effectively filled in and the new tunnel bored from scratch, hence there is
no large-diameter junction tunnel there, though the change in tunnel lining
is obvious.


Also, judging by the size of the southbound platform, I'm assuming that
this
was where the original station was, and that the old northbound line was
covered up by the platform.

Correct. The station had a narrow island platform, like those which still
survive at Clapham Common and Clapham North.

If this is so, do the original NB tunnels still
exist behind the walls at the ends of the platform?

Yes. They are accessible through doors in the headwalls at both ends of the
(now) s/b platform. At the southern end of the station there used to be a
siding (though which part of the n/b diversion tunnel runs), and the
junction tunnel associated with that also still exists.