On 08/09/10 12:57, tim.... wrote:

How can you run out of credit "on" the bus?

Well, you can discover that you've run out of credit after you've
boarded and the bus has already departed.

On a bendy bus, you board the bus and then use one of the Oyster readers
inside the bus to validate your card - by which time the bus has already
departed.

-roy