In article , Barry Salter
writes
For a Thameslink journey to, or through, Farringdon, Barbican and
Moorgate your ticket MUST have Underground Zone 1 validity. A ticket to
London Terminals isn't valid beyond City Thameslink from the South and
St Pancras/Kings Cross Thameslink from the North.

Just to confuse the issue further: one can buy tickets from stations in
Thameslink North (e.g. Luton) to those in Thameslink South (e.g.
Gatwick) which are routed "Thameslink only" i.e. they are not valid for
cross-london travel via underground, nor at any U1 station, but only via
Farringdon etc. I'm not sure at what point the rules change - if you ask
for a ticket from Luton to a destination in south London within the
travelcard zone system, you tend to get a zonal ticket as being the
cheapest. But other ticket types must surely exist, e.g. for travel in
peak hours.

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Clive Page