Definition of "unscheduled"

There were two further unscheduled stops, one at a small station, Skalice, where a small party of tourists entrained, and at Břeclav, a junction and customs post on the Austrian frontier.

1947 January and February, Gerald Druce (Jun.), “A Journey on the "Slovak Arrow"”, in Railway Magazine, page 17

Because most diesel failures can be traced to electrical faults, minor in themselves but often difficult to pin-point, any unscheduled halt during a trial run is often the signal for the frenzied unfolding of wiring diagrams and the appearance of an impressive array of voltmeters and circuit testers.

1962 April, R. K. Evans, “The Acceptance Testing of Diesel Locomotives”, in Modern Railways, page 268