State officials are seeking to expand commuter rail and extend the Green Line even as ridership figures indicate suburbanites are slowly abandoning the existing public transit options available to them.

The troubling trend line has transit advocates worried. Most of them say deteriorating service is to blame for the downturn in ridership, while others speculate that commuter traffic is down because more and more suburbanites are exchanging their green lawns and picket fences for urban living.

“This is a relatively recent phenomenon of declining growth and we wonder: Is it the beginning of a new trend?” asks Stephanie Pollack, associate director of the Kitty & Michael Dukakis Center for Urban and Regional Policy at Northeastern University. “The system is definitely more about putting out fires than expanding service. It’s really getting hard to use the system.”

Since 2003, overall ridership on all MBTA-operated commuter systems is up 13.2 percent. But the bulk of that increase has come from the three “heavy rail” Orange, Blue, and Red subway lines and the bus system, which combined have seen ridership increase nearly 20 percent in that period. The three subway lines themselves have increased more than 24 percent.

The numbers for several other modes of transit have fluctuated in the last 10 years with periods of growth offset by declines, some related to the economy, some to fare increases. But even though some have shown increases the last couple years, they are still below the peaks they hit during the past decade. The Green Line, for instance, showed slight growth in 2012 over 2011 but that followed decreases in four out of the previous five years. In 2012, nearly 76 million people rode the Green Line, down more than 7 percent from the high of 82 million riders in 2007.

Ridership on commuter boats started strong but didn’t keep growing. In 2003, 1.36 million commuters took the MBTA boat service. Last year, 1.34 million people rode the seas to work.

According to figures from the American Public Transportation Association, Massachusetts commuter rail lines in 2003 carried 40.3 million passengers. By 2012, that number dropped to 36.4 million, a decrease of 10.4 percent. Those figures include the addition of the Greenbush line on the South Shore as well as several new stops on the Fairmount line. Without the riders added at those new stops, the decline would be even more stark.

In the first two quarters of this year, traffic on the Red, Orange, and Blue lines was up less than one-half of 1 percent. Ridership on all other forms of public transportation, including commuter rail, the Green Line, and boats, was down. Commuter rail ridership is down 5.6 percent, Green Line traffic is off 9.6 percent, and boat ridership fell 13.2 percent compared to the same period last year. Overall, ridership on MBTA-operated public transportation is down 3.8 percent so far this year.

T spokesman Joe Pesaturo says part of the cause of declining ridership is the fare increase instituted last year. He said the dropoff in ridership has been less than had been expected.

“The fare increase was a primary factor,” he wrote in an email. “A pre-fare increase impact analysis had projected a ridership decrease of as much as 5.5 [percent]. Despite the ridership decline, the MBTA collected more than half a billion dollars in fares for the first time in its history.”

The decline raises questions about the need or the appetite for expansion of service such as South Coast Rail from Boston to Fall River and New Bedford and the Green Line extension into Somerville and Medford. Pesaturo says Greenbush has failed to meet expectations because there are other options for those who live along Greenbush, such as the boat or driving to Red Line stations in Braintree and Quincy, though the boat numbers would seem to undercut that argument. Pesaturo says the lack of similar alternatives for those in New Bedford and Fall River would make that line a success.

Former Secretary of Transportation Frederick Salvucci says it’s not clear exactly what the numbers mean, but he says it is clear that the age of the state’s transit infrastructure and equipment is having a negative effect on service and, in return, ridership.

Salvucci, a professor at MIT, said when the original commuter rail contract was signed with the Massachusetts Bay Commuter Rail company to manage the system, there was an agreement to buy new equipment. But, he said, a suit by disabled T riders forced the agency to spend money on accessibility upgrades in other areas, which delayed the purchase of new locomotives. The T agreed to hold MBCR harmless for equipment failures beyond the company’s control, but that agreement has done little to reassure customers trying to get from home to work and back in a timely manner, says Salvucci.

“The equipment the MBCR is using is crummy, so there are lots of service failures that are not held against them,” says Salvucci. “They’re doing okay in terms of meeting the contract terms. They’re not doing okay in terms of the service as seen by the customers. It gets disrupted very often. . .Service is less good from the perception of the customer even if they [MBCR] are meeting the terms of the contract.”

Salvucci says one theory floated by an MIT colleague may have some validity in light of the data. He said his colleague believes that more people are moving from the suburbs back into Boston, boosting transit traffic in the city but reducing the inflow from outside. Salvucci says the reverse migration could explain the uptick in city-centered transportation, such as subways and buses, and the downturn in traffic on commuter rail and boats.

“His theory is what we’re seeing is a lifestyle change where more and more people who work in the city are choosing to live in the city. They sold the house in Concord and picked up an apartment in Beacon Hill or Jamaica Plain or Cambridge or Arlington,” says Salvucci. “It’s an interesting theory. It strikes me as unlikely that kind of societal change would happen this fast but it might be worth a study to explain these numbers.”

Northeastern’s Pollack, who rides the commuter rail from Framingham and is a staunch advocate of public transit, says the T does a woeful job of marketing itself and attracting new riders or retaining old ones. But she said all those bad numbers could be easily overcome with upgrades in service and equipment.

“People shouldn’t have to choose between using transit and having a pleasant commute,” she says. “Transit can sell itself as a superior way. What it can’t be is worse.”