Nearly two months ago, Metro set up "express" Farecard machines at five rail stations in an attempt to improve the transit authority's trouble-plagued fare collection system. The experimental devices accept only $5 bills and dispense $5 Farecards.

So far, officials said, the move appears to have fallen short of expectations. Although the machines have been largely trouble-free, the blue-paneled devices have been used by relatively few rail patrons.

"The machines are not being used as expected," said Fady P. Bassily, Metro's rail service chief. "They are not carrying their fair share."

The number of $5 Farecards purchased at the express machines has ranged from 375 a week at the Rhode Island Avenue station to 1,500 weekly at Silver Spring. But ordinary Farecard machines are used far more often -- 4,000 to 5,000 times a week at the five stations.

Metro officials said they plan to continue testing the express machines until late next month before making further recommendations. One proposal currently under study is replacing them with $10 express vendors -- a plan that might require Metro to purchase special equipment.

Some patrons say they avoid the $5 machines because they prefer to buy Farecards worth $10 or more, Bassily said. In addition, he said, some passengers complain that the $5 machines fail to provide a bonus. Ordinary Farecard machines offer a 5 percent bonus with Farecards of $10 or more.