Trolley motors are usually run on DC current.  Now days, they use solid state devices, called rectifiers, to convert standard AC to DC current.  Back in the olden days, they needed a "motor dynamo" sets to make the conversion.  An AC motor would turn a DC Generator, like pictured here at the Yakima Trolley Museum.

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See inside
Yakima Trolley
car.

In a nutshell, here is why I think they did this in so many places.  DC motors are used on trolleys and buses because it is easier for them to speed up and slow down.  They can even go in reverse with ease, if need be. 

AC is the current of choice for the power company because its voltage can be stepped up for high voltage transmission lines as well as stepped down for safer house current.  AC to DC conversion is often needed.
My NW bicycle tour 2002

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