Running underneath Euston Road is this secret tunnel that joins one private part of Regent’s Park to another. Apparently it was used by the folk who lived in the nearby fancy houses, and who wanted their children to play in the park, but didn’t want them crossing a busy road.
The black curved iron at the bottom of the picture looks like it might be part of the Bakerloo tube line, which has a station a few minutes up the road. Amazing to think about what goes on under you feet.
Edit: a visitor has informed me that it’s the Metropolitan line that runs underneath the tunnel.