Grand River Transit

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This is the only Route 91 clue that I could find from a cached copy of the CBTDB Wiki from May 4, 2022. The first entry about the route was from November 2012 and "Late Night Loop" is used before that:

91 Late Night Loop is a conventional bus route operated by Grand River Transit. The route serves Charles Street Transit Terminal, Grand River Hospital, Uptown Waterloo, Wilfrid Laurier University, University of Waterloo, and the student-dominant communities of Lakeshore and Columbia.

This route operates on Thursday, Friday and Saturday late nights only.

The fact that it started at the Charles Street Transit Terminal would suggest that the driver and bus was pulled from another route which ended at the terminal at 1:00am. I can't remember if Route 9 connected to the Transit