The buses I’m familiar with that use overhead power lines don’t use them exclusively - they have batteries or diesel for when they venture off the powered lines.
Interesting, I've seen them stuck more than once, I'll have to check up on what the deal is there, I walk by the depot every couple of days and I'm sure they'll be happy to talk about it.
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.
