If you’re too young to remember, a trolley looks like a bus, rolls
on tracks like a railroad car, is powered from overhead wires,
and is usually found in cities. (Today, very few cities use
trolleys.) Trolley systems were also called “interurban”.

Transportation and technology provides us with some of the weirdest
coinage of words. Trolley is no exception.

A trolley normally picks up electricity from a long pole on the roof.
The spring-loaded pole has a wheel on the end that rolls on an
electrified overhead wire. This wheel is called the troll, thus
trolley was named after this little wheel.

So where did troll come from? The Welsh troell means wheel, reel,
pulley, windlas, etc. The old French troul, trouil (treuil) means
reel, winch.

So how did the word troll get linked to rotating devices?

Animals that had a certain saunter or gait gave the appearance of
rotation to their motion. Also, hounds on the move in packs
would sometimes swarm or swirl in a sort of rotary motion.

The old French word for this motion was troller (trôler): “hounds
to trowle, raunge, or hunt out of order”. (from Cotgrave, R., an
old French-English dictionary, 1611)

So . . . trolleys ultimately got their name from swirling dogs!