It is full of gigantic loud spinny things. It is amazing. You must go.
The museum is small, but it's also the actual head-end of the cable car system. The museum part is a mezzanine overlooking the workshop floor and the giant wheels that run the four remaining cable car lines. The current plant was built in the early 80s, but earlier in the century there had been dozens of plants around town to run the various lines, each powered by steam engines. Steam engines!
It's also a great place to get your apocalyptic car-hate on, once you read about how the majority of the cable car lines were dismantled due to lobbying from the then-powerful internal-combustion bus lobby, despite the fact that cable cars were cheaper and more reliable. The busses mostly won, obviously, but there was a public outcry that saved a few of the cable car lines. San Francisco has been the only place in the world with an operating cable car system since 1957.