I was standing on the westbound central line platform at holborn
yesterday, and I noticed that the western end of the platform has this
wierd jutting out bit over the tracks, a bit like a very thin bridge
over the tracks.

Considering that the entrances to the platform are at the ends of the
escalators, and centred on the middle of the platform, the western end
of the platform must be very close to Kingsway.

The central line is only one flight of escalators, so it must only
just be lower than the Kingsway tram tunnel - could the bridge-like
thing on the platform be the bottom corner of the tram tunnel?

Makes me wonder whether the door at that end of the platform leads
into it, or could it just be a disused interchange with the picadilly
line platforms?