In message , at 13:16:27 on Fri, 3 Mar 2017,
tim... remarked:

No, you might get an unauthorised overdraft on your credit card,
and quite likely a penalty charge from the card company.

if you got these it wouldn't be rejected, would it?

The two outcomes are mutually exclusive.

Not if the Card Company sends a flag back to TfL saying "that was at
your risk because the customer is overdrawn".

and what on earth does that mean?

A transaction can be accepted in full, accepted at the retailer's risk,
or refused completely.
--
Roland Perry