>How does the machine know there will be a cap by credit card? How does it track
>Why people don't use a contactless credit card to avoid that
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Residents visiting from outside the TfL zones usually get a zones 1-6 combo National Rail Day Travel-card, so don't need any-further payment method,
Some travellers have season tickets for their journeys.
Some concession type Railcards (Senior/16-25/disabled) can't be programmed on contactless bank cards, but they can be set on an Oystercard
Not all non-UK issued Contactless debit cards work and there may be overseas bank fees.
Some people set up Apple Pay.
Oyster/contactless tracks because you tap in and out on the underground and use single-tap flat fares for buses.
Oyster has been around for 14 years, contactless technology is much newer, TfL introduced it about 3 years ago, as mentioned not everyone has a contactless card yet or wants one, in some countries their banking system and technology is much behind so cards are not issued.
Once you have an Oystercard it's worth creating an account, you can view all your journey history for the previous 8 weeks and see how your fares/capping are calculated, you also have protection against loss or theft.