John Rowland wrote:

Is that the beautiful but sad staircase in a cage?

Yes, that's it. This subway served platforms from 9 to 12/13. From
what I can make out from old drawings, 3/4/5 did not exist before the
rebuilding for the Central Line, so the long subway under 3/5 was
probably built at this time. This subway led to the stairs up to the
platform, just above the words 'New Westbound' on the new platform
level drawing. There was a doorway to the left which led onto the
low-level, and a short subway to the right serving platforms 6, 7
(never used) and 8. During the last rebuilding this subway was
extended as ffar as platform 12, and the old subway closed. For a while
both were open, and you could make a circular walk. Finally, the new
subway to the West of the low-level lines, serving 3-10 was opened.
What is now 9/10 was extended a considerable distance, it used to end
not far from where 10a does now. I am not clear how what is now 6/8
was reached prior to 1949, mayde there was a stairway from the old
subway behind one of the closed doors. This platform was also much
shorter until the time of the electrification and Central Line
extension.