The new Transbay Transit Center probably won’t open until June, but the first bus service from the new transportation hub will start the day after Christmas.

Muni’s decision to run some of its buses on the 5-Fulton route from the terminal’s street-level bus plaza, beginning Dec. 26, is part of an effort to break in the sprawling new transit hub.

The partial service start also appears to be an effort to satisfy a federal loan requirement that the terminal reach “substantial completion” and begin Muni service by the end of 2017.

The first buses to pull out of the still-under-construction terminal will be on short-run routes bound for the Richmond District. Instead of going all the way to Ocean Beach, they turn around at Sixth Avenue and Fulton Street. Those buses run from 7 a.m. to 7 p.m. on weekdays.

Muni decided to start service gradually as a way of getting used to the new terminal and working out details and difficulties before seven other lines move in, along with other transit agencies, in the spring, said Erica Kato, a spokeswoman for the Municipal Transportation Agency.

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“It’s almost a pilot of sorts,” she said.

The $2.3 billion transit center, with an empty train station in the basement and a verdant park on the roof, was supposed to be operating by the end of the year with buses from a half-dozen transit agencies rolling into service in early 2018.

Construction had remained on schedule until this spring, but in October, the expected grand opening of what some have called “the Grand Central Station of the West” was pushed back, possibly until June, because of delays in installing nearly 50 miles of cables.

“Our hope is to start operations sometime between the end of March and June,” said Mark Zabaneh, executive director of the Transbay Joint Powers Authority, which oversees the terminal.

Until then, the 5-Fulton is likely to be the only route running in and out of the Transbay, he said, though some transit agencies may run buses through the station for training purposes when construction allows it.

When the transit center opens, it will serve only buses. Muni will operate its 2-Clement, 3-Jackson, 5-Fulton, 5R-Fulton Rapid, 7-Haight-Noriega, 38-Geary and 38R-Geary Rapid lines from the lower-level bus plaza.

On the elevated bus deck, two levels above ground and connected to the Bay Bridge by a swooping bus-only ramp, Muni’s 25-Treasure Island bus will join AC Transit’s transbay buses, the main terminal tenant, along with WestCAT, Amtrak and Greyhound. SamTrans and Golden Gate Transit buses will stop curbside outside the terminal.

On Tuesday, the first buses ran through the terminal in an early test run. Muni ran buses on both service levels while AC Transit tried out the bus deck.

In addition to bus service, the 2½-block-long terminal at Fremont and Mission streets will be home to shops, restaurants, administrative offices, waiting areas and a 5.4-acre rooftop park. It also has a vacant basement with room for rails, platforms and a future train station. It’s intended to eventually house a Caltrain extension and high-speed rail.

In the meantime, Muni’s 5-Fulton service will make its final stop on Beale Street between Market and Mission streets, dropping off the last inbound passengers. Then it will head into the terminal, where it will pick up outbound passengers.

The 5-Fulton route was chosen, Kato said, because it also runs from stops at the Temporary Transbay Terminal at Beale and Howard streets. Muni ambassadors will be on hand to direct riders to the new stop at the Transbay Transit Center.

“One of our priorities will be to maintain a convenient transfer between AC Transit transbay lines and Muni lines,” she said.

Michael Cabanatuan is a San Francisco Chronicle staff writer. Email: mcabanatuan@sfchronicle.com Twitter: @ctuan