Reports of smoke at the Downtown Crossing station triggered massive disruptions to the MBTA's core subway system, shutting down more than a dozen stops on a day when the morning commute was already snarled by winter weather.

Shuttle buses replaced Red Line and Orange Line service at more than a dozen stops in the area Tuesday morning. Stretches of the Green Line in the same area were also offline for a multi-day, pre-planned closure.

The T reported the smoke at 8:22 a.m. Tuesday, as problems began for commuters returning from a holiday weekend. At 8:36 a.m., the T said on X that passengers at JFK/UMass station could use the commuter rail to get to inbound locations.

At 8:46 a.m., the T reported that buses were replacing train service between JFK/UMass in Dorchester, though Boston, and to Harvard Station in Cambridge "due to an electrical problem at Downtown Crossing."

"Boston Fire Department and our maintenance personnel are on scene. Updates will be provided," the T said.

The problems expanded to the Orange Line shortly after 9:30 a.m. Also citing an "electrical problem at Downtown Crossing," the MBTA said on X that shuttle buses replaced Orange Line trains between Back Bay and North Station.

Green Line shuttle buses were extended to run between Copley/Back Bay Stations and North Station to accommodate additional passengers, the T said.

By about 10:45 a.m., the T had announced that regular service had resumed on both the Red and Orange lines, initially with delays.