Ridership at the Massachusetts Bay Transportation Authority has declined in recent years, a common story at transit agencies across the country and one that T officials have told often in the last year.
But lurking under Boston Harbor is a major exception, linking downtown to the growing populations in East Boston and Revere.
The Blue Line has bucked the trend with a rush-hour ridership spike of about 18 percent since 2014, officials said this week. Even off-peak ridership, where the T has seen most of its losses, is up by more than 10 percent on the Blue Line.
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That puts it alone among the T’s subway lines, as the Red, Green, and Orange lines — plus the bus system — have all seen ridership dips in recent years.
“Clearly, something is going on with the Blue Line,” said MBTA analytics director Laurel Paget-Seekins.
Officials have posited several theories for why system ridership is decreasing, including increased competition from Uber and Lyft, lower gas prices, and bus routes that haven’t kept up with population changes.
Likewise, they offered several possible reasons the Blue Line is growing: new housing developments in East Boston, the neighborhood’s unique geography, and the fact that — unlike its siblings — it has room to grow, with the lowest subway ridership in the system.
Indeed, Paget-Seekins cautioned that on the Blue Line, an increase of a few thousand riders a day has a proportionally outsized impact. A similar increase on the Red Line would be a “drop in the bucket,” she said. The Blue Line has about 63,000 riders a day, compared with 244,000 on the Red Line, and about 190,000 each on the Green and Orange lines.
Nonetheless, the new ridership on the Blue Line is notable, Paget-Seekins said, adding that the harbor may be one reason. Residents along the line essentially have just two options — take a train or a car — if they want to avoid making the long, roundabout trip downtown using some other transportation mode, such as a bicycle.
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MBTA surveys show that 50 percent of Blue Line riders sometimes use an alternative to the subway, compared with 53 percent on the overall subway. And while 11 percent of subway riders said they bike as an alternative, only 4 percent of Blue Line riders said so.
“It’s more competitive with other forms of transportation,” Paget-Seekins said. “Especially [because of] the inability to bike.”
But Paget-Seekins added that ridership seems to be growing more at the Revere terminal in Wonderland than other stations, suggesting that more North Shore riders are driving and parking at the station’s large garage. She did not have a theory for why.
The off-peak passenger increase is especially striking, since ridership on the Red and Orange lines has dipped outside rush hour by 9.2 and 5.6 percent, respectively. Logan International Airport, which can be accessed by the Blue Line, may be keeping ridership flowing, both from travelers and from workers with non-traditional hours.
It is typically the most reliable on the subway system, boasting a 94 percent on-time rate in the last month compared with 89 percent systemwide.
Eric Bourassa, transportation director for the Metropolitan Area Planning Council, a regional agency, noted there has been an uptick in residential development in East Boston and Revere, with at least 1,800 housing units added since 2010. According to US Census estimates, East Boston’s population increased by nearly 6,000 between 2010 and 2016, while Revere’s increased by about 1,500. And the two communities are plotting several big developments, highlighted by one at the old race track at Suffolk Downs, that could further spur population growth.
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But there’s also been plenty of recent development along the other lines that hasn’t stemmed the ridership drop. The Blue Line’s biggest advantage, Bourassa said, may just be that it has more room for new riders. The Red and Orange lines, especially, are typically packed to the point of discomfort in the morning rush, a poor experience that could deter growth.
“Is it because the Blue Line has more capacity, and the rider experience feels better?” Bourassa wondered, contrasting it with the Red and Orange lines. “If you show up and it’s so crowded that you can’t get on the train, is that enough for people to say, ‘I don’t like this anymore and I want to change my behavior?’”
Paget-Seekins said the T’s major projects on the Red and Orange lines — like improving signals and replacing train cars — will boost capacity on those lines, allowing them to add more riders in the future. Officials are also planning changes to the Green Line to add capacity, including new, longer trolleys.
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But with the Blue Line on the rise, she said, the agency is beginning to consider improvements so it can also take on more passengers.
East Boston’s city councilor, Lydia Edwards, said she expects ridership on the line to keep growing. She said the T must prepare to increase the frequency of trains, and she called on the agency to extend the line to Charles/MGH Station, where it could connect with the Red Line. State officials are considering that idea but have not committed to it.
“It’s not enough to have an increase in ridership if they’re still hitting the same roadblocks,” she said.
Adam Vaccaro can be reached at adam.vaccaro@globe.com. Follow him on Twitter at @adamtvaccaro.