You may find it hard to believe, considering the terrible commutes of the past few days, but a few MBTA lines — or at least the vehicles that run on them — have become significantly more reliable in recent years.
Trains on the Red and Orange lines, as well as the agency's fleet of buses, have broken down noticeably less often, data from the T show.
Jeffrey D. Gonneville, the T's chief operating officer and its former chief mechanical officer, said the public transit agency has focused on finding and fixing numerous "low-hanging fruit" problems that were responsible for large numbers of breakdowns.
"We're targeting the things that are giving us the most trouble and getting those things repaired," he said.
Trains on the Green and Blue lines as well as commuter rail locomotives, meanwhile, have not done as well, posting stagnant or slightly worse results in the past couple of years, though all were at least slightly better compared with five years ago.
The T tracks the reliability of its trains, trolleys, and buses by measuring the average distance traveled before the vehicles break down.
The statistic, called mean miles between failures, accounts only for vehicle breakdowns, like the kind that triggered a chaotic, smoky scene at State Street Station Tuesday night when an Orange Line train failed.
The data omits other kinds of breakdowns, such as delays in service caused by track issues, medical emergencies, police activity, or signal problems, like the one that caused widespread problems on the commuter rail Thursday.
The agency has set different goals for how far vehicles should travel on average between failures:
• The Red Line goal is at least 47,000 miles
• The Orange Line goal is at least 37,000 miles
• The Blue Line goal is at least 35,000 miles
• The Green Line goal is at least 5,500 miles
• The goal for buses is 12,250 miles
• The commuter rail goal is 10,000 miles
Two lines — the Green and the commuter rail — fell short of their goals last year, while the other lines surpassed their respective targets for 2015.
The following charts show how often vehicles on each line have broken down on average each year, going back to 2010. (Remember that higher mean miles between failures is a good thing, indicating better vehicle reliability.)
Gonneville said the T has launched major initiatives to repair portions of its aging fleet.
For example, he said, the agency is working on a pair of projects to refurbish dozens of Red and Green line cars that were built in the late 1980s.
The T also has put about 40 new commuter rail locomotives in service in recent months and, over the next several years, it plans to replace hundreds of Red and Orange line trains and dozens more on the Green Line.
Gonneville said he expected the new trains would "act as reliably, if not more reliably," than the current ones. The Globe reported this month that some new commuter rail locomotives have had problems, though they have been generally more reliable than the old ones.
Along with age, weather can be a common cause of vehicle breakdowns, Gonneville said.
"Certainly the winter weather [of 2015] took a toll on all of our equipment," said Gonneville.
Cold temperatures can cause moisture buildup in pneumatic air systems to freeze and cause breakdowns.
Hot weather can also pose problems. Propulsion equipment can overheat, and air conditioning and ventilation systems can fail.
"The spring and fall months tend to be the better months for performance here at the T and for other public transit systems that have temperature fluctuations like we do here," said Gonneville.
The Globe reported last month that T vehicles broke down more often than most other transit systems in the country during 2014. That analysis was based on data collected by the Federal Transit Administration, which followed a different definition of mean miles between failures than the one the T uses. The federal statistics are not available for 2015.
Another metric the T uses to track the reliability of its system — the percentage of passengers who did not have to wait on the platform for longer than the scheduled times between trains — indicates that the worst performing line in recent years has been the Orange, followed by the Red and then the Blue. The Green Line is not yet tracked using this statistic, but the T plans to do so soon.
Gonneville acknowledged that the figures are not perfect. He said the agency is working to develop new metrics that do a better job of capturing how timely and reliable service is.