Buses will replace trains between Ronkonkoma and Brentwood on the Long Island Rail Road this weekend, the railroad said.

The ongoing work is part of the $387 million double-track project that is expected to increase capacity along the 18 miles between Farmingdale and Ronkonkoma when it is completed at the end of next year.

The work this weekend will affect service from 12:37 a.m. Saturday until 12:21 a.m. Monday, the railroad said.

Westbound riders at Ronkonkoma and Central Islip will board a bus to Brentwood, then transfer to a train for service to points west. Buses will depart up to 34 minutes earlier than usual train times.

The 10:05 a.m., 11:05 a.m. and 12:05 p.m. westbound trains from Ronkonkoma will originate at Farmingdale. There will be no bus service from Ronkonkoma for these three trains. Riders at stations east of Farmingdale should board an earlier or later train (or board a bus at stations Ronkonkoma and Central Islip).

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Greenport-Ronkonkoma trains will depart 21 minutes later than usual on Saturday and Sunday. At Ronkonkoma, riders will transfer to buses for service to Brentwood, where train service resumes.

Riders were advised to plan for up to 60 minutes of additional travel time.

Eastbound trains on the Ronkonkoma Branch will terminate at Brentwood, where riders will board buses for service to stations at Central Islip and Ronkonkoma.

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Medford-Greenport riders will also connect with buses at Brentwood, the railroad said.