Train service on the Ashmont leg of the Red Line was halted this 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.

"A permanent repair will be made tonight. The cause remains under investigation," Johnson said.

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, but it is not yet clear if the below-freezing temperatures played a role in the defect. 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," Johnson said.