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Long escalators lead to the platform of the newly-constructed Central Subway MUNI station at Union Square in San Francisco.

An art installation titled “Lucy in the Sky” hangs above the entrance to the newly-constructed Central Subway MUNI station at Union Square in San Francisco.

A train tunnel is seen under construction in the new Central Subway MUNI station platform at Union Square in San Francisco.

San Francisco’s decades-in-the-works Central Subway extension has been delayed once again, this time by about two months, the Municipal Transportation Agency said Tuesday.

A June 20 fire in the subway’s Yerba Buena/Moscone Station is expected to delay completion of the Central Subway by six to eight weeks, Jonathan Rewers, SFMTA’s acting chief financial officer, told the agency’s Board of Directors. The fire didn’t result in any injuries or damage to trains, though it temporarily halted testing inside the Central Subway, one of the final steps toward the project’s completion.

A surge arrester ignited a fire inside a circuit breaker cubicle and damaged two other cubicles — all three of which need to be replaced, Rewers said.

While the SFMTA said last year that it projected the Central Subway would open in October, agency officials later broadened the timeline to sometime in the fall and have been hesitant to announce a firm opening date.

“We’re still planning for revenue service this fall,” Rewers told the board.

The Yerba Buena/Moscone Station where the blaze happened is one of four new stations that will serve South of Market, Union Square and Chinatown. Once it opens for service, the T-Third Street rail line, which merged with the K-Ingleside line during the pandemic, will no longer take riders into the Market Street subway.

Instead, the T line will take riders to the four new stations from the stop at Fourth and King streets. The Central Subway’s Union Square/Market Street station will connect to Muni and BART’s Powell Station via an underground concourse.

Originally scheduled to open in late 2018, the Central Subway will tentatively open four years behind schedule.

The subway project’s completion will signal the beginning of a transit expansion lull in San Francisco. No major transit projects are under way following the opening of the Central Subway and the delayed but popular Van Ness Bus Rapid Transit project.

City planners are studying expanding rail transit, possibly BART, westward on Geary Boulevard and 19th Avenue. Tunnels inside the Central Subway extend to North Beach and planners have studied potentially extending the subway line north to Fisherman’s Wharf.

Ricardo Cano is a San Francisco Chronicle staff writer. Email: ricardo.cano@sfchronicle.com Twitter: @ByRicardoCano

Ricardo Cano covers transportation for The San Francisco Chronicle. Before joining The Chronicle in 2021, he covered K-12 education at CalMatters based in Sacramento and at The Arizona Republic in Phoenix as the newspaper's education reporter. He received his bachelor's degree in journalism at Fresno State.