Made a special trip to Wuppertal during the summer to have a go on these. They're good fun, work really nicely. Getting a picture that looks this natural is an achievement, though - it's a really industrial part of Germany, you get a lot of views of lagged pipework.
Nitpick: I think trolleys are specifically street cars, which Wuppertal unfortunately dismantled several decades ago. Wherever I look they're referred to as trains. This specific one is the Kaiserwagen ("Emperor's carriage / wagon" is probably the most fitting translation), which you can rent out for events. They're not used for regular transportation.
1823, in Suffolk dialect, "a cart," especially one with wheels flanged for running on a track (1858), probably from troll (v.) in the sense of "to roll."
So I'm gonna go with your definition - trolley has wheels. Ugh, dammit, this has wheels on top.
Maybe trolley in the sense of usage? As in, used like a trolley or streetcar?