Paying for Thameslink trains is highly flexible, especially within London and the Southeast. The most convenient method is Contactless payment using your credit card, debit card, or mobile device (Apple/Google Pay); you simply "tap in" at the start and "tap out" at the end. You can also use an Oyster card for journeys within London’s Zones 1–6 and as far as Gatwick Airport or Luton Airport Parkway. For longer journeys outside the Oyster/Contactless zones, you must purchase a paper ticket or an e-Ticket via the Thameslink app or website. Another modern option is the Key Smartcard, which allows you to load tickets or "pay as you go" credit electronically. It is vital to remember that Thameslink is a "penalty fare" zone, so you must have a valid form of payment before you board, as you cannot pay the conductor once the train has departed.