John B wrote:

AIUI, all FCC trains that terminate in King's Cross during weekday
daytime hours will run through the Thameslink tunnels after the
project, leaving KX for GNER, Hull and Grand Central services only.

I'm not sure what will happen at weekends and evenings (when FCC local
services currently also run into KX instead of Moorgate).

Presumably there will be capacity in the current suburban trainshed, once
vacated by outer suburban services.