Update: Since this story published, San Francisco Muni has decided to extend the boat tram’s season. The vintage rail car will operate Tuesdays and Wednesdays through Fleet Week, October 6-14.

Imagine if someone took a chainsaw to the roof of one of San Francisco Muni’s vintage street cars, opened it to the sky, then strung party lights from a central cabin and hung a pirate flag from the stern. You’d have, essentially, a topless trolley, or one of San Francisco’s two boat trams — social media stars of Muni’s fleet.

The nonprofit Market Street Railway, which acquires and preserves historic railcars for San Francisco, launched an @boattramsf Twitter account in May, and the inanimate vehicle’s online presence has quickly emerged as a delightful respite from the 280-character fray.

What exactly does an 85-year-old trolley do on social media? Post glamour shots (“feeling cute might delete later”), retweet selfies from new friends, promote boat tram season (“#hotboatsummer”) and generally remind Muni passengers that they can ride a boat … on rails … along the Embarcadero … for free.

Built in 1934 to cruise the waterfront avenues of Blackpool, England, boat trams have been operating in San Francisco since the 1980s, when Market Street Railway acquired one for Muni, shipping it across the Atlantic to its new coastal home. (Despite the trolleys’ appearance, they are not amphibious vehicles, and alas, could not sail independently into San Francisco Bay.)

The tram proved popular enough that Market Street Railway purchased a second car in 2013. Today, the trolleys are minor celebrities of the Muni system, running along the Embarcadero on Tuesdays and Wednesdays during the summer, and inspiring folks to hop aboard for a free lift — destination: “nowhere in particular,” according to the marquee signage. (The trams actually run between the Ferry Building and Pier 39.)

The final day of regular summer operation was Sept. 4, but passengers have two more chances to live the boat tram life during Muni Heritage Weekend Sept. 7-8, when one of the railcars will be out on the streets from 11 a.m.-5 p.m. Saturday and noon-5 p.m. Sunday alongside its vintage partner.

Should you make a special trip just to cruise on an odd hybrid boat trolley for a few blocks along San Francisco Bay? Absolutely. Riding one is like stepping into an alternate reality where public transportation is an unmitigated joy and it feels like the operators and passengers might break into a spontaneous (but tightly choreographed) song at any moment.

Or, as boat tram conductor Roderick Mills explained on a sunny afternoon between welcoming passengers aboard and ringing the departure bell at every stop, “This is like the Disneyland of street cars. People get on here and they’re like 5 years old. It’s amazing how good people feel when they see this train and get on it.”

The boat tram is a mood. A really good one.

Sarah Feldberg is a Chronicle staff writer. Email: sarah.feldberg@sfchronicle.com