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.