Originally Posted by BnaBreaker
That makes a lot of sense! Thank you Electricron for that very precise and detailed answer.
Electricon's answer is right on...except, as he pointed out, when it's not, which is why it's such a tough question to answer.
(1) Light rail trains don't share lanes with other traffic while streetcars will
... except that some light rail does operate in shared ROW.
(2) Light rail trains can use multiple units while streetcars don't.
...typically true, but many streetcar vehicles can be coupled into at least 2-units.
(3) Streetcars can turn tighter curves than light rail trains.
...can and should are not the same thing. Light rail typical is 82-feet, streetcars you can ratchet down to 50-feet... but then, light rail can theoretically go tighter. And most design specifications you'll see for streetcar systems call for the light rail standard anyways (less wear and tear on the vehicles; higher operating speeds). But this is an important factor in vehicle selection, so it's as good of a criterion to use as any.
So like anything else, the answer is, it depends. The light rail/streetcar distinction is very much an American thing, and as I think Cirrus pointed out, it stems from our habit of using light rail as a "metro light." But that is by no means necessary - plenty of places mix and mingle the two vehicle types operationally.
In the U.S., I think the easiest distinguishing characteristic is shared ROW versus not. That might require us to re-think some of the older light rail systems, but so what?