"Surely the clearances arn't *that* tight? Besides , didn't they
extensively rebuild that part of the tunnel in the last few years?
Perhaps they improved the clearances while they were at it and these
restrictions no longer apply. "

Whilst work may have happened on the southern end of the new
Thameslink station box at St Pancras, where the track is relatively
straight, I am not aware of any work on the reverse curves (the Hotel
Curves) between KX Thameslink and the new station box. These are, I
believe, the main constriant to the route clearances.

BTW how do you improve clearances in a brick tunnel under a builidng,?

OC