The frustrating slowdowns are a symptom of the T’s aging infrastructure and track defects that make it unsafe to go faster. The Red Line slowdown in Quincy, for example, was needed because of defective fasteners that secured the rails underneath the tracks. The current Green Line crawl is due to a number of issues, including bad ties and a water leak that corroded the tracks between the Boylston and Arlington stops.

Transit agencies usually impose speed restrictions on certain portions of tracks when it is unsafe for trains to run at their normal speed. Most transit systems consider slow orders temporary measures until repairs are made — not long-term patches. But an over-reliance on speed restrictions, experts say, is a sign that an agency isn’t keeping up with needed repairs.

“They’ve created their own mess here,” said Keith Millhouse, a rail safety expert based in California. “Had they done their job, they wouldn’t be in this position.”

T spokesman Joe Pesaturo acknowledged that the length of track covered by such speed restrictions has increased, but blamed it partly on increased construction activity on the tracks.

Discrepancies between its internal and public data, he added, may be because some reports don’t count restrictions like weather. Pesaturo wouldn’t say why those possible omissions were not disclosed.

Part of the T’s problem, by its own admission, is that much of its own speed restriction data is unreliable. For years, the agency tracked individual slowdowns on a basic spreadsheet that could be edited retroactively. It began compiling more thorough weekly reports this August, and a more sophisticated tracking system is supposed to be completed next year.

Tracking this data is key. The number and duration of speed restrictions are a widely used measure of a transit system’s health, and mass transit systems rely on speed restrictions as a way to thwart potential problems and keep riders safe.

“The point of a slow order is to ensure safety,” said Allan Zarembski, a professor at the University of Delaware and director of its rail engineering and safety program. “It’s a lot less unhappy than a derailment or having something else bad happen.”

After a restriction is imposed, transit systems should move quickly to address the problems underneath, he added. “You can’t continue doing the same old, same old and expect to catch up.”

Though there is no national standard for how much of a system should be under restrictions, such orders are meant to last weeks or months at the most, said Millhouse. “If you’re using them for any other purpose, you’re not using them correctly,” he said.

Instead of making badly needed upgrades, the T has forced trains in several places to travel slower for months and even years, the new data show, quietly inconveniencing travelers without saying when the issues will be fixed.

That’s left many T riders in the dark, like Ashmont resident Sylvia Broude, who regularly commutes on the Red Line.

”There are moments on my commute where the train feels like it is crawling forward, moving so slowly on the tracks — I can only guess that’s because of the track conditions,” said Broude, who has at times turned to biking or driving, out of frustration.

“As a rider, it just feels really unreliable,” she added. “I wish I had more information.”

The lack of transparency erodes more than the track, said advocates; it also chips away at riders’ trust in the T.

“It’s entirely reasonable for a transit system to have various slow zones throughout the year. It’s common to do repairs,” said Stacy Thompson, executive director of the LivableStreets Alliance. “The reason it’s become a thing is because the T isn’t transparent about why.”

The T is already facing a crisis of confidence, after a bruising year of prominent safety incidents, federal scrutiny, and rising public concern about its reliability. This August, federal investigators singled out the T’s pattern of long-running restrictions, including the persistent, years-long slow zone along the Orange Line downtown between Tufts Medical Center and Back Bay stations.

The Federal Transit Administration found that this spring, nearly 10 percent of the MBTA’s Red, Orange, and Blue subway tracks were under speed restrictions, in addition to 2 miles of Green Line track. Track maintenance crews usually have only a 2- or 2½-hour window to complete fixes overnight, which federal investigators slammed as not nearly enough time. The Green Line work train used for maintenance had been inoperable for at least eight months, further hindering improvements, the FTA found.

Shortly after those findings, the T imposed an unprecedented monthlong shutdown of the Orange Line to make badly needed repairs. Upon reopening the line, the T declared victory, saying it had “eliminated” six slow zones. But it took weeks after the reopening for the MBTA to actually finish repairs and lift some restrictions, and some — like the stretch between Tufts Medical Center and Back Bay stations — are still not at full speed.

The T’s data show that despite such efforts, speed restrictions and their underlying problems have lingered across the system.

A review of 60 speed restrictions that were in place across the system in October show that more than half were because of track problems — and more than 20 percent had been in place for at least a year, according to one internal T document.

Many of the slowdowns were necessitated by problems common to an older transit system: rusting rails, bad fasteners, or tracks tilting out of shape.

This fall, the T estimated that the total lag time on the system — the time added by having to slow down the trains — varied from 30 minutes to more than an hour. And the T is falling behind on fixing restrictions, from a backlog of just 14 in 2020 to 55 in 2022.

Pesaturo, the T spokesman, said despite logistical challenges and supply issues, the agency had made progress in addressing some restrictions, like removing slow orders on the Blue Line in early November and improving times on the Orange Line after the monthlong shutdown. Six speed restrictions on the Red Line have been lifted south of JFK/UMass, though four remain.

“Because much more work will take place next year, the MBTA has already started the process of procuring shuttle bus service providers for future diversions,” he added.

But in addition to undermining a system’s efficiency, procrastinating on repairs can make them more difficult to complete later, said Al Fazio, a rail safety expert.

“The deterioration curve gets steeper,” he said. “The worse it gets, the faster it gets worse.”