unexplained
adjEtymology From un- + explained.
1
Not explained; of unknown cause or origin.
- Why this already very fast train should be speeded up still further, when none of the other more easily timed S.R. West of England trains has a single minute pared from its schedule, is unexplained - unless this is a playful dig at the Western Region, most of whose expresses, by reason of additional stops, will be decelerated from the same date.1961 October, “The winter timetables of British Railways: Southern Region”, in Trains Illustrated, page 593: