The MBTA tweeted an apology to its riders this morning after disabled trains on three subway lines snarled the morning commute.

“We apologize for delays this AM,’’ the agency agency tweeted on its official Twitter account, @mbta.

In its tweet, the agency included a link to a press release from Governor Deval Patrick announcing the bidding process has begun for new trains for the Orange and Red lines, which are currently projected to be in use in the winter of 2018 and in the fall of 2019, respectively.

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T spokeswoman Kelly Smith said that the T’s chief mechanical officer does not believe this morning’s disabled trains can be attributed to overnight cold. Temperatures in Greater Boston this morning were about 20 degrees colder than Thursday morning, dropping into the low 30s, according to the National Weather Service.

Smith said the cause of the breakdowns won’t be known until after mechanics examine the individual trains sometime today.

Trains were disabled on the C Line of the Green Line and at the Alewife Station on the Red Line and at the Oak Grove station on the Orange Line.

Smith said the T communicated the snafus to riders through the T Service Alert program, which communicates via text message to individual riders’ cellphones, and not through the agency’s Twitter account.

She said the T staff responsible for posting tweets is currently reduced to one person, leading to the decision to rely on T Service Alerts as the primary means of communication this morning.

The T also notified riders about delays over loudspeakers at the stations and at signal boards at stations throughout the system. The T also posted to its service alerts page on mbta.com, officials said.

Delays due to signal problems were also reported on the Fairmount, Providence/Stoughton and Lowell commuter rail lines today, according to the service alert page on mbta.com.