The only ones I have spent a lot of time looking at are in Seattle and at least they used to all be diesel and could hookup to the lines or lower them to run off the lines. For pure electric I assume they must tow them away but I don't know enough about the failure modes.

This is a good chance to share a link to the guy with a Prius modified to run on the bus power in San Francisco: https://thebolditalic.com/hacked-prius-running-on-muni-power...