Shifting gears from ferries:
What kinds of obstacles -- whether they be physical, regulatory, or otherwise -- preclude LRT vehicles from using HRT tracks for non-revenue moves? Prompted by discussion in the
If I Were God/Goddess thread, I was thinking about what it would take to run a small LRT line on the North Shore. Obviously, when you reach a certain size for an LRT subnetwork, you should just build a yard and run the thing independently. But what if it's just a short line?
What I'm thinking about is whether a non-revenue connection could be built between LRT "Urban Ring" tracks and Blue Line tracks near Wood Island or Airport, and then run LRT cars out-of-service up the Blue Line outside of peak hours to then run on, like, a Lynn-Marblehead service or something. As far as I know, both sets of overhead catenary use the same voltage, and it's the same gauge, and all of the curves should be doable for LRT if they are doable for HRT.
The other place I've thought about something like this is Lexington. I like the idea of extending the Red Line to Arlington Center or Arlington Heights, but it seems less convincing to go further. The lower density of Lexington could be friendly to an LRT line -- but if you build RLX, how do you connect to the the larger LRT network? Assuming a GLX to Porter with a non-revenue extension to Alewife, and assuming it's possible to run LRT cars over Red Line tracks outside of peak, then you could run a short LRT line through Lexington without needing to build new yards. (I think.)
The only place that I know of right now that runs LRT and HRT on the same tracks is Cleveland (though I understand they're abandoning that with their next set of rolling stock).