I've made another serendipitous discovery on Wikipedia!
The noun 'bustitution' is a portmanteau of 'bus' and 'substitution', and can be used to refer to the temporary or permanent replacement of train services with bus services.
It has given rise to a transitive verb, 'to bustitute', and an adjective, 'bustituted'.
While we're on this topic, I have to say that the phrase 'rail replacement bus' annoys me. Why not 'train replacement bus'?
4 days ago