[Reason077]

London Underground and London Buses are both "semi-online". They don't generally try to immediately validate a transaction with your bank the moment you touch your card. Transactions are batched and applied overnight, after applying any discounts like daily/weekly fare caps, out-of-station interchanges, and the bus "hopper fare". The batching means that transactions can still be accepted if the terminal is offline for some reason.

However, there is also a blacklist of card numbers that have outstanding balances against them. If you try to use a card that is declined, it will work the first day but not on subsequent days. If you go on TfL's website and clear the outstanding balance, the card will work again after 30 minutes (or in practice, less) once it is removed from the blacklist.