When I was a kid in Des Moines in the 1940s, I remember the automatic streetcar system in-street turnouts where lines would diverge upon leaving the downtown area. When a car approached, they somehow "knew" which track it was supposed to follow and positioned themselves accordingly. The "switch" in the overhead line did the same thing so the trolley followed the proper wire. I've always wondered what kind of sensor was employed for such automation to properly align the turnout and overhead. How did it "know" which line that particular car was supposed to follow? Can someone explain?

(When the automatic function failed to do its work properly, the motorman had to use a large "pry-bar" to move the track points and then manually move the trolley to the proper overhead wire. I remember how the motorman would stand up and look down through the windshield at the switch as the car approached to be sure it was positioned properly. And even if it was, sometimes the overhead didn't set correctly and the trolley would go off on the wrong wire.)