If you're lucky enough to get Wifi on the tube, or enough personal space to move your arms enough to check your balance, you might see that TfL has charged you before the end of your journey, likewise with Uber.

This is typically a small amount like £0.10 or £1, and is simply their way of checking that the card you're using is real and you're actually able to pay for your journey. 

Don't fret, it's not an additional charge, either the same day - or a few days later - the amount will "settle" and the full charge will appear in your account.

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