Roland Perry wrote:

I see that this is creaking into existence once more (it's only been
postponed three times already). Will trains from the GN lines continue
to terminate at Kings Cross, until the Thameslink works are complete, or
will some of them start going through?

The map on the National Express Group website shows the GN and existing
Thameslink routes as continuing to be self-contained, albeit coming
tantalisingly close at KX and Moorgate:

http://www.nationalexpressgroup.com/...hise_route.pdf