"Roland Perry" wrote

There has been much redevelopment in the area, including the railway
line going "underground" much earlier (as it emerges from Blackfriars)
and the construction of City Thameslink, whose northern exit is pretty
much where Holborn Viaduct used to be.

I haven't been there for some time, but when St Pauls/City Thameslink
opened, the northern exit went through the concourse of the former Holborn
Viaduct station.

Peter