Police activity on the tracks of the MBTA Green Line B Branch was causing long delays Wednesday night, the T said.

It wasn't immediately clear what caused the activity on the train's right of way, at South Street station. The MBTA only noted that trains might stay at stations, with delays estimated at 25 minutes.

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South Street is near the Chestnut Hill Reservoir.

This is a developing new story that will be updated with more information.

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