What is a Streetcar?

Platonic thought experiment meets transportation

Last month I offered this question/thought experiment on Twitter:

Addison Del Mastro @ad_mastro

What do you think of a streetcar as being conceptually: a train that rides on the street, a bus that uses rails, or something else?

I like questions like this. I thought about similar questions here—seeing hugely spread-out street grids in suburban road networks—and here, looking at the gradual evolution of the strip mall out of the traditional urban block.