Buses are replacing some off-peak trains on the Montauk branch as of Monday because of rail testing, the Long Island Rail Road said.

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Sections of track are beingtaken out of service for testing through Thursday, and special schedules are available at terminals and stations or online at mta.info/lirr.

The testing is done by the Sperry rail car, which detects internal rail defects not readily visible to the eye. Defects that are found are corrected immediately by a crew of LIRR track maintenance workers, the LIRR said in a news release.