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 "motor dynamo" sets to make the conversion.  An AC motor would turn a DC Generator, like pictured here at the Yakima Trolley Museum.

Images taken 2002.
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.

Another piece of civic pride is the Yakima Trolly, said to be the oldest continuously operating trolly to run between two cities in USA.  Runs from Yakima to Seala, WA.  This trolly kept operating through the late 1960s and early 70s because it also hauled fruit cars from orchard to warehouse.  This was enough justification to keep it alive until it started being used as a historic tourist oriented line.

Most old trollies stopped operation and then started up again as historic lines.  Yakima kept going until it became history.  A friend of mine, who lives in Yakima, volunteers for the trolley line.

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