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?

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

Many thanks,

Daniel.