Yes, the TfL system does a partial authorisation. It checks the card is valid and not blocked etc but doesn’t necessarily do a real-time authorisation all the way to the issuing bank.
If you try to use a card that is valid but has no available balance/credit, it might work for the first ride but then be blocked when you try to use it for the return trip.
Fares are batched throughout the day and you are charged once, overnight, for all rides that day (after applying any multi-ride discounts, etc).
This is different from some other cities where I’ve used contactless payments and they’d charge you immediately for each ride, giving you lots of annoying little charges on your bank statement!
Wonder how they block the card, my impression was that tokenization was meant to make it harder for card chargers to be able to track a card through multiple taps like that.
TfL not only gets your Apple Pay device account number (DAN), but can also associate it with the primary card number (PAN). Both magically appear if you add the primary card to your online TfL account and have used the associated Apple Pay device with TfL before.
If you try to use a card that is valid but has no available balance/credit, it might work for the first ride but then be blocked when you try to use it for the return trip.
Fares are batched throughout the day and you are charged once, overnight, for all rides that day (after applying any multi-ride discounts, etc).
This is different from some other cities where I’ve used contactless payments and they’d charge you immediately for each ride, giving you lots of annoying little charges on your bank statement!