It is the beginning of the end of paper rail tickets for some Southern passengers travelling into London.
Commuters using the Brighton line into the capital will be able to use smart-cards, similar to Oyster-cards, at East Croydon, Clapham Junction, London Victoria and London Bridge.
The scheme will be widened out to all London stations as of next year.
Baroness Kramer launched the new scheme yesterday.
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Tuesday night's weather for the east of the region
Tuesday night's weather for the west of the region