A rechargeable smart card alternative to paper tickets is being officially launched on the Southern Railway line between Brighton and Seaford.
A pilot scheme has been under way for nearly a month, with about 100 passengers and railway staff using "the key" and card readers at stations.
Transport minister Norman Baker is demonstrating the card at Lewes station in his East Sussex constituency.
It is to be offered to more passengers soon after the three-month trial ends.
The key, which is similar to London's Oyster card, is charged online or at self-service ticket machines to pay for train journeys.
Card readers have so far been installed at Brighton London Road, Moulsecoomb, Falmer, Lewes, Southease, Newhaven Town, Newhaven Harbour, Bishopstone and Seaford stations.
It had been intended to start the trial in September but testing was extended to deal with teething problems.
Southern said the Brighton to Seaford line had been chosen for the pilot because it was the only operator on that section.
It also had a good mix of passengers, including students, season ticket-holders and leisure travellers.
It hopes to introduce a joint smartcard with Brighton and Hove Buses and Metrobus by September 2012.
London's Oyster card readers will not accept them until December 2013.