While this might seem to make the machines of limited value, transit officials argue that they are a significant improvement because the dirt, along with the garbage, contributes to track fires. The M.T.A. hired Dominique Renaud, an expert on transit system vacuums, to help design the new machines.

“If you’ve noticed, in between the rails at many stations now compared to what it was years ago, that’s all gone now,” Joe Lhota, chairman of the M.T.A., told reporters recently. “There used to be all kinds of soot. You can actually see the ties now.”

Since December, the two shopping-cart sized vacuums have filled about 69,000 bags — some 1.7 million pounds of dirt — from all of the E line and parts of the F and A lines.

In just two recent nights, a total 2,411 bags were removed from the Church Avenue station in Brooklyn.

Since the vacuums made their debut, track fires have decreased, from 75 in January to 53 in June.

On a recent Thursday night, a team of about 20 workers met at the Jamaica Center-Parsons/Archer stop in Queens to pick up one of the machines.

Jan Frankel, 50, who supervises the team, described the vacuum cleaner as the “best thing that the M.T.A. has ever done.” Beside decreasing track fires, he said he hoped the push to more thoroughly clean the subway would convince the public to respect the tracks and stop littering.