THE MBTA’S OWN DATA indicate passenger traffic on the Green Line is declining, but agency officials say they don’t believe the numbers.

Jeffrey Gonneville, the T’s chief operating officer, told the Fiscal Management and Control Board on Monday that the Green Line is the nation’s busiest light rail line, providing more than 200,000 trips each weekday.

But Gonneville also included a chart in his presentation that showed that Green Line traffic has fallen since fiscal 2008, fairly dramatically by one measurement. He said the number of average weekday unlinked trips on the Green Line has fallen from more than 250,000 in fiscal 2008 to about 190,000 this year. He said card tap-ins at Green Line stations and on Green Line vehicles had declined more moderately over that period, falling from about 140,000 to about 120,000.

Gonneville said the Green Line traffic numbers are generated using actual fare data plus extrapolations, and the information is reported to the federal government. He said agency officials don’t believe ridership has fallen as dramatically as the numbers indicate, if at all.

Transportation Secretary Stephanie Pollack asked whether it was fair to say the T doesn’t know how many people ride the Green Line. Gonneville called Pollack’s question “relatively unfair,” but acknowledged the T will be scrutinizing the numbers much more closely in the future.

Overall, Gonneville’s presentation on the state of the Green Line wasn’t very positive. He said delays are common, with 20 to 30 percent of riders experiencing delays in February and March. Getting people on and off the trains is a major problem on above-ground routes. He said the system also suffers from old, mismatched trains from different decades that have difficulty communicating with each other. He said collision-avoidance signal systems on the Green Line are little more than traffic lights.

After major crashes in 2008 and 2009 that resulted in one fatality and many injuries, the MBTA, at the urging of federal regulators, investigated the installation of a system that would automatically keep Green Line trains at safe distances from each other. The T ultimately concluded it would take nine years and $700 million to install such a system, which was so cost prohibitive that the agency is currently exploring alternative safety approaches.

Gonneville also said the T is going to consolidate four stations on the B branch of the Green Line along Commonwealth Avenue.  He said the preferred distance between stops on that stretch of the Green Line is 1,200 to 1,400 feet, but many stops are much closer to each other. With Boston officials preparing to redo a section of CommonWealth Avenue, Gonneville said the T plans to use the opportunity to consolidate stops at Babcock Street, Pleasant Street, St. Paul Street, and Boston University West into one stop.