Service on one of Boston's subway lines was interrupted on Thursday morning when what firefighters described as a small fire was reported underneath a train car and they had to manually carry water up to an elevated platform to fight it, authorities said.

The operator of a northbound train on the Red Line's Charles/MGH Station saw smoke coming from the undercarriage area just before 6 a.m., a spokesperson for the Massachusetts Bay Transportation Authority said in an email.

Passengers got off the train and evacuated the station, and there were no injuries reported.

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"We received a call for a fire on the Red Line outbound tracks. The companies arrived and they found a small fire on the tracks on the outbound side with people self-evacuating," fire Deputy Chief James Greene said at the scene.

However, the MBTA said there was no confirmed report of flames or fire. The smoke emanated from an air compressor control box on the train, spokesperson Joe Pesaturo said.

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The water supply system on the platform malfunctioned and firefighters had to manually carry cans of water to the platform, Greene said.

Shuttle buses replaced Red Line service while MBTA personnel moved the disabled train, and service resumed just before 8 a.m., with trains bypassing Charles/MGH. The station is expected to reopen later Thursday.

The MBTA has had a series of problems and accidents in the past several years that prompted federal intervention.