I totally get with the prepaid oyster that when it’s topped up I’m physically touching it to the contact so the data is transferred, but with a debit card (especially if I’ve say paid for something online) how does it know what’s on the card ?)
In: Technology
It does not. The way it works is that it will always allow you to tap in if your card is not blocked by the TfL systems.
After a few seconds it will charge you £0.10 to make sure that your card is active. On the next day it charges you the relevant amount for the travel you have done.
If at any point they try to charge you and you have insufficient funds the card details go into their block list and is not allowed to be used again unless your debit is paid.
By doing it this way they ensure very fast processing and the only risk for them is someone using a card that does not have enough funds for a day’s journeys and never use that card again. So in the bigger scheme of things, minimal risk.
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