The MBTA red line has been slowed for weeks headed northbound to Harvard Square from Central Square. (Photo: Marc Levy)

MBTA crews are working overnight this week to replace a rail causing trains to slow to a crawl in the red line stretch from Central Square to Harvard Square, said Joe Pesaturo, director of communications at the Massachusetts Bay Transportation Authority.

The 10 mph speed limit heading northbound between the stations was imposed last month after a track inspection discovered a hazard, Pesaturo said in an email exchange Wednesday and Monday. Another slowdown was implemented this month southbound from Davis Square to Porter Square; repairs on that stretch was not mentioned by Pesaturo.

The difference in speeds is noticeable, with trains rolling forward quietly in the seven-minute drag from Central to Harvard, then barreling loudly from Harvard to the station at Porter.

“With safety of paramount importance, the MBTA decided to implement a speed restriction,” Pesaturo said by email. “The MBTA apologizes for the temporary adverse impact on commuting times and appreciates its riders’ patience while track maintenance personnel schedule and complete the work needed to enhance the safety and reliability of the infrastructure.”

The transit agency’s engineering and maintenance division had been working to schedule an appropriate time to perform rail replacements, Pesaturo said. This week it pinned down the work hours, though Pesaturo did not say exactly when the repairs were expected to be complete – or even the days the slowdowns began.

The rail system has been bedeviled by a greater than usual number of problems for months, starting in June, when federal inspectors said the MBTA had too few people to operate at weekday levels. That shifted the subway system to a less-frequent Saturday schedule, and a  shutdown of the entire orange line followed Aug. 19 to Sept. 18, with a partial shutdown of the green line tacked on. 

The transportation advocacy group TransitMatters released a statement Tuesday expressing exasperation with the MBTA’s lack of transparency on problems and repairs.

“Anyone riding the rails between Alewife and Ashmont or Braintree has almost certainly noticed that their ride is also quite a bit slower than it should be. The TransitMatters Slow Zone Tracker shows that a round trip on the red line is almost 15 minutes longer than it should be, with no sign of improvement,” the group said. “The MBTA must come forward with a complete and specific description of the deficiencies on the red line and be fully transparent about what it will take to fix them and whether the T has the resources to do the job.”

The group calls on the T to give at least three months’ notice of closings and diversions and provide high-quality alternatives; be clear about the work being performed and the condition of the infrastructure, before, during and after the diversion; and that all work be done overnight or on weekends, with early closings rather than full shutdowns such as the approach taken on the orange line.