Train service on the Ashmont leg of the Red Line was halted Tuesday morning after a “track defect” was detected during the chilly morning rush-hour. Commuters had to board shuttle buses for about one hour as workers hustled to make a temporary repair. Train service resumed around 9:40 a.m. with cars running “at limited speeds,” according to MassDOT spokesman Jason Johnson.

Southbound service was suspended from 8:30 a.m. to 9:40 a.m. Both directions were impacted during the repair process from 9 a.m. to 9:40 a.m., Johnson said.

The delays came on the coldest day of the winter to date and initial assessments by T officials indicated that the below-freezing temperatures were the likely cause of the defect.

“The exact cause remains under investigation, but it is believed to be related to the extreme drop in temperatures from fairly mild to bitterly cold,” said Johnson. “Such a drop can be very stressful on a running rail.”

A joint bar, similar to a brace, was installed on Tuesday morning to preserve the integrity of the rail until permanent repairs was made after the service day ended on Tuesday.

The MBTA suspended service on multiple weekends last year to conduct “winter resiliency work” on the Red Line, but much of the work was focused on the Braintree leg south of JFK-UMass.

“The MBTA completed an enormous amount of work throughout the entire system in just a few months,” Johnson said. “In the spring, the MBTA will continue to make improvements with work on the Dorchester branch of the Red Line.”

“The MBTA apologizes to its customers that were adversely impacted by this morning’s track problem on the Red Line. We will continue to work very hard to deliver the level of service our customers expect and deserve,” said Johnson.

The brief shut-down on the Red Line was not the only problem on the MBTA system on Tuesday, a day after MBTA overseers advanced two options for hiking fares this year by roughly 6.7 percent or 9.7 percent.. A commuter rail train without any passengers derailed north of the Andover station The two derailed coaches - which did not tip over - clogged railroad commutes north of Boston as it tied up trains farther up the line. Keolis canceled some runs and used buses to transport riders between Haverhill and Andover.

“The track structure failed. We don’t really know why at this point,” Rich Murphy, director of train operations for Keolis, said in a video posted online Tuesday morning explaining the delays.

Including the derailed train, five trains were trapped north of Andover as a result of the accident, according to Keolis.

Reporting by the State House News Service contributed to this story.