My partner and I did part of the Lea Valley Walk, yesterday, between
Hatfield and Hertford, starting at London. We got single tickets; we
would have been cheaper to use returns to Stevenage.

On our way back, a Hatfield to London Terminals ticket was valid at
Moorgate after we got the Underground at Finsbury Park - those NR
services do not run to Moorgate at the weekend.
The ticket was returned by the machine; I started to wonder what worked.
I then found the following:

OUT IN Station
Y - KXSP NR
- Y KXSP LU
N - Farringdon
N - Barbican
Y Y Moorgate
N N Liverpool Street

As it happens, my investigations were backed up by a Z1-3 Travelcard and
the ticket stopped working at Moorgate after it had been rejected at
Liverpool Street; it worked at Moorgate after rejection at Farringdon.

A previous ticket from St Pancras to Harpenden showed as London
Thameslink to Harpenden. It seems that London Thameslink consists of St
Pancras, Farringdon, City Thameslink, Blackfriars, and London Bridge.
It is notable as permitting exit at Farringdon which London Terminals
never did.
--
Walter Briscoe