The frigid weather played unpleasant tricks with public transportation today, disabling subway and commuter rail trains and leaving passengers frustrated. Amtrak said the cold would also force service reductions Friday on its lines running south from Boston.

Five disabled subway trains on the Red Line, at Braintree, Porter Square, Quincy Station, Fields Corner, and between Wollaston and Quincy Center, caused delays on both the Ashmont and Braintree branches between about 6 a.m. and 8:30 a.m., according to an MBTA spokeswoman. A disabled train on the Mattapan high-speed portion of the line at Cedar Grove caused delays at around 7:15 a.m.

On the Green Line, two disabled trains, at Kenmore and Reservoir stations, caused delays at around 6:50 a.m. and 8:15 a.m.

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A disabled train at Back Bay Station on the Orange Line caused delays at around 7 a.m. Yet another train was disabled on the Green Line in the afternoon.

“Most of the delays were moderate to severe, but mostly moderate,” said spokeswoman Kelly Smith, who said the disabled trains were caused by “the effects of the extreme cold temperatures on our aging equipment.”

Crews were working to repair the disabled cars, she said.

The Massachusetts Bay Commuter Railroad Co., which operates commuter rail service for the MBTA, said two trains were delayed today because of the cold.

Amtrak said its Acela Express and Northeast Regional lines would operate on a modified schedule on Friday because of the “extreme cold.”

“Passengers should expect reduced frequencies and plan for the possibility of delays,” Amtrak said in a statement.