Last night the through Gatwick Express to Brighton was cancelled due to a fault in the class 442 unit that normally run the service.
A class 460 unit was drafted in, which terminated at Gatwick, where a 4-car unit for Brighton was waiting at the far end of the same platform. Passengers were told to transfer and the train left as soon as people were ready, arriving at Brighton only ten minutes late.
It was all painless and a model of good operating. That's the way to do it. But this is only possible if there is some slack in the system to enable the operators to respond when things go wrong.