I used to do 1st-line IT support in a branch of a department store chain. One of the calls we dreaded taking was a faulty till in the smaller of the two customer-catering outlets within the store, as they had only one till.
When this happened, the assistants would say to customers to just sit down and have their drinks/etc as normal and pay on their way out; if it still wasn't working on their way out, then if the customer was paying cash payment would be taken and a hand-written receipt issued, but if the customer was paying by card, no payment would be taken.
We could've made the customer walk to the another catering outlet in the store (on a different floor, and the other side of the building), but given the choice between irritating a customer and losing £5-£10, there was a clear direction from management to write off the small amount of money to keep the customer's future business - I think this approach was right.
As this was the smaller (coffee/cake) catering outlet there weren't any instances of over £10 being written off - if there had been, I think we would've asked them to go to the main restaurant to pay, but would've given them a voucher for a free coffee/cake next time to make up for the inconvenience
This thread is discussing a similar set of circumstances, and not only are people saying its right to expect the customer to trek to an ATM and back, but also that they shouldn't be trusted to do so without being followed. It is not unreasonable for a customer to expect to be off the train and out of the station within 5 minutes, so if the railway company can't do this due to equipment failure, they rather than inconvenience their customers and make them late for whatever appointment, bus connection etc they should consider letting customers off with small amounts (e.g. under £10) and sending bills out for larger amounts.