Mike Bristow wrote:

The longest Tube journey is on the Central Line, West Ruislip to
Epping, at 54.9 km (34.1 miles).

Can't you do Cockfosters - Cockfosters via the Heathrow loop that would
be
longer?

If you're lucky you can probably do somewhere-in-west-london and
back again on the central line via it's loop - or do they turf
passengers out of those trains?

I can't actually recall seeing services advertised on the indicators as
running through to Hainault-Woodford (other than on days when part of the
loop is out of service and the rest is being treated as a standard branch
from Woodford) and the last time I was at Hainault the train terminated and
a different one came in for Woodford.