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www.sfzoo.org]
tl;dr: dates back to at least 1905, ran at a World's Fair (not in SF), Santa Cruz, eventually came to the Zoo funded by Mr. Fleishacker. Ran for many years, with a newspaper noting that it was the "only train in SF that runs full on every trip." After the original track layout became Gorilla World, it lived with elephants for a long time in storage. Eventually, the equipment and cars were pulled out and restored, and a new track was built (GGRM was part of the crew). PG&E provided part of the funding, so of course the locomotives were converted from the original coal to "clean-burning natural gas." If you visit the Zoo, the train still runs there; take a ride.
I did just that a couple of years ago. Nice ride. Track needs work.