A correction has been added to this story. The T says it computes time savings based on roundtrips, so a 33-minute improvement would be on an end-to-end roundtrip of 140 minutes, not 70 minutes as the original story suggested.

MBTA GENERAL MANAGER Phillip Eng, who has come under fire for failing to tell riders when their commutes will improve, took a significant step in that direction at Thursday’s board of directors meeting.

In recapping the T’s plan to shut down the Red Line between JFK/UMass and Ashmont stations for two weeks in October to perform track work, Eng promised that trip times would improve significantly.

“When we are done with this work through the end of November, it is anticipated that we will be giving back a total of 42 minutes to our riders – the majority of that is on the Red Line,” he said. He added that Red Line commute times would drop by 33 minutes while commute times on the Mattapan Line, which is also being shut down for repairs, would drop by five minutes.

Since the Red and Mattapan line work is scheduled to run from October 14 to October 29, it wasn’t clear what exactly Eng was saying with his reference to the end of November. It also wasn’t clear how time savings of 33 minutes on the Red Line and five minutes on the Mattapan Line would add up to 42 minutes of total time savings.

T spokesman Joe Pesaturo did some checking and provided some clarity. He said the work on the Ashmont Line and other yet-to-be-announced work planned for November would cut travel times on the entire transit system by 42 minutes. The biggest time savings – 33 minutes — would come on the Red Line.

With all of the current slow zones, Pesaturo said, it currently takes more than 70 minutes to travel the Red Line from Alewife to either Ashmont or Braintree at the southern end of the line, a distance of less than 20 miles. He said the travel time for an end-to-end roundtrip is expected to be reduced by 33 minutes, a drop of 24 percent, after track work is completed by the end of November.

“There is much work to be done on the sections of the Red Line that do not include the Ashmont branch,” Pesaturo said in an email. “The 33-minute projection is for a train that does an end to end roundtrip. Today a trip from Braintree to Alewife is more than 70 minutes.”

Cutting a 140-minute round trip on the Red line to 37 minutes is a major improvement, and it’s unclear why the MBTA and Eng aren’t leading with that information.

Eng has been criticized for failing to lay out a plan or timetable for improving service levels. Boston Globe columnist Joan Vennochi raised that issue on Wednesday, former state transportation secretary Jim Aloisi did it last weekend in CommonWealth, and CommonWealth’s Michael Jonas did it last month.

Eng’s brief comments on Thursday don’t come anywhere close to laying out a master plan for fixing the T. But, once deciphered, they represent one of the first times the T has tried to spell out in concrete terms what repair work would mean for riders.

“We’re doing what we promised to the public,” Eng said on Thursday. “We’re giving them back invaluable time in their day that they’ve lost to us.”

Now the T has to deliver.