I had read that there were some shelters built under some of the
central tube stations, but that the planned one at St. Pauls was never
actually built.

Now, I was passing through St. Pauls this morning on an eastbound
train, and happened to stand at the back (west), which is unusual for
me, and when we got to St. Pauls, I noticed that there seemed to be a
series of several black doors at the west end of the platform. I would
guess this is pretty close to where the old post office building was,
near the bombed church, and there seems to be a wierd blockhouse in a
similar position in the middle of the road on the surface.

Did they build it after all, as it would have been good for the Post
office, but keep it secret, or are these doors something completely
unconnected?