This is the Down Moorgate Line, where trains climbed up the slippery steep slope of Hotel Curve from the Widened Lines. It is way over on the West Side - Platform 13 if my memory serves me right.
I think we mentioned this on another thread, where I posted the picture below.
You can't see this view of the entrance anymore as it is under the West Side development - though I have a sneaky feeling that the tunnel remains and is in use, as there is a significant amount of commercial traffic which goes down a new concrete slope from Goods Way and disappears in the direction of Kings Cross undercroft.
