Throughout this site there are some words and phrases that have specific meanings.  These are explained here.

Non-TfL Rail:  The train services within zones 1-6 which joined the Oyster system in January 2010 including the pilot stretch between Victoria and Balham.  These are Southeastern*, Southern (except between Clapham Junction and Watford Junction), South West Trains, First Capital Connect (except between Elephant & Castle/London Bridge and West Hampstead, and between Kings Cross/Moorgate and Finsbury Park), and National Express East Anglia (except between Liverpool Street and Seven Sisters/Tottenham Hale/Walthamstow Central/Stratford).

TfL Rail:  The underground, DLR and train services within zones 1-9 which joined the Oyster system before January 2010 (apart from the pilot stretch between Victoria and Balham).  These are London Overground, London Midland between Euston and Watford Junction, First Great Western, Heathrow Connect*, Chiltern, First Capital Connect between Elephant & Castle/London Bridge and West Hampstead, and between Kings Cross/Moorgate and Finsbury Park, National Express East Anglia between Liverpool Street and Seven Sisters/Tottenham Hale/Walthamstow Central/Stratford and C2C between Fenchurch Street/Liverpool Street and Grays (but not at Maryland or Forest Gate).

National Rail: All National Rail services* (including London Overground) where part of the journey is using Non-Tfl Rail.

* Oyster Cards are NOT valid on Southeastern High Speed services between St Pancras International and Stratford International, Heathrow Express services or Heathrow Connect services between Hayes & Harlington and Heathrow Airport.