In article . li,
Tom Anderson wrote:
There is an easement that allows doubling back at Kings Cross, so you
can go from FPK to KGX to pick up the fast train,

Ah, i didn't know that. Can you double back by tube?

Tube and rail are interavailable between FPK and KGX IIRC, so I would
imagine in principle yes, but whether the LUL gatelines and/or staff
know agree is another matter.

(N.B. The doubling back rule is for journey from FPK to specific
destinations - I'm not suggeting there's a general overarching
permission to double back at KGX)

-roy