The modest improvement nonetheless marks a significant milestone on the T’s journey to returning the sluggish line to full speed, which will require more upgrades.

The MBTA had promised faster trips as the big payoff for riders who endured the unprecedented shutdown from Aug. 19 until Sept. 19. But the “slow zones” that plagued the line before the shutdown have persisted long after the reopening, and some still remain.

With fully functioning tracks, Orange Line trains should be able to travel up to 40 mph along most of the 11-mile line, T officials have said, except along curves where maximum speeds are 25 mph.

In June, federal inspectors issued a scathing report about widespread defects on MBTA subway tracks and urged the agency to fix longstanding issues in the Orange Line slow zones — areas where trains could not go the normal speed limit because of damaged or outdated equipment. Inspectors from the Federal Transit Administration began enhanced oversight of the MBTA in April after a series of safety incidents, including the dragging death of a Red Line passenger that month.

MBTA officials said track work completed during the Orange Line shutdown was supposed to improve travel times once the new track parts had time to settle, no later than one week after the line reopened.

But after a series of conflicting and confusing explanations for the ongoing speed restrictions, MBTA General Manager Steve Poftak told US Senator Edward Markey in a letter this week that the repairs were insufficient to immediately eliminate speed restrictions in many areas.

Of six slow zones the MBTA said it had eliminated on Sept. 18, three now require trains to travel below full speed, and three are at full speed, according to agency spokesperson Joe Pesaturo. In two of the remaining slow zones, speeds have improved since the shutdown — between Tufts Medical Center and Back Bay stations and between Stony Brook and Jackson Square stations — while one is slower than before — Assembly to Wellington stations.

The T has also improved speeds in areas of track outside of the six slow zones it targeted during the shutdown, including between Assembly and Sullivan stations and near Ruggles Station, according to Poftak’s letter to Markey.

In his letter, Poftak said most of the remaining speed restrictions will be lifted in November and December after the MBTA completes more work, including replacing rail and ties.

The MBTA does not publish subway speed restrictions, leaving riders to guess how long they will last, and has repeatedly declined to provide a list of them to the Globe. TransitMatters uses MBTA data to monitor train travel times and identifies areas as slow zones after four consecutive days of delays.

And it’s not just the Orange Line that’s running slower, as riders know all too well. TransitMatters’ slow zone tracker shows a roundtrip on the Red Line was more than 10 minutes slower Thursday than if trains were running at full speed, worse from in mid June, when a roundtrip was nearly three minutes slower.

Stacy Thompson, executive director of LivableStreets Alliance, said the MBTA needs to be more transparent with “metrics that matter to the public,” such as work to eliminate speed restrictions and hiring efforts to improve train and bus frequency.

“Does the T still need to get their act together to explain to riders what they’re doing to make it safer and better? Yes. That hasn’t meaningfully changed,” she said.

Pesaturo, the T spokesperson, did not say when the Red Line will be running at full speed. The agency replaced rail near Harvard Station and between Porter and Davis stations in the overnight hours this week and will be doing “rail maintenance work” between Broadway and Ashmont stations this weekend, when shuttle buses will replace subway service, he said.

“The MBTA appreciates its riders’ patience while workers make these important safety improvements,” he said by e-mail.