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CAMBRIDGE - When leaves start falling this autumn, riders on the commuter rail may see something new on the tracks - an 89-foot flat car with two large black tanks spraying a high-pressure stream of water onto the rails.

The Massachusetts Bay Commuter Railroad Company, which operates the commuter rail for the MBTA, introduced this week its second high-pressure rail washer for the coming foliage season to clean up "slippery rails" covered with oily fallen leaves.

"The problem is when the leaves settled on the rails and the train rolls over, the compaction creates the oily residue that gives the traction problem," said Richard Davey, deputy general manager of the commuter rail company.

A black ooze from the leaves is left on the rails, making them too slippery for wheels to gain traction. Typically in October and November, train operators need to brake earlier to make stops and accelerate more slowly to limit wheel slippage.

Still, they sometimes overshoot train stations. That can add to passengers' commute times and leave them waiting and late for work and school.

And, when the slippage happens repeatedly, it eventually damages the train itself. Excessive braking and skidding flattens train wheels, and eventually requires realignment of the wheels - a process called "truing."

"The problem is that the cost is very expensive. You have to take the trains out of service," said Scott Farmelant, spokesman for the commuter rail system. "You take the wheel out and reformat it so that it fits on the rail perfectly.

"It takes time and costs money, and also causes inconvenience to customers."

Slippery rails are a problem throughout the Northeast. The Southeastern Pennsylvania Transportation Authority and New Jersey Transit use similar high-pressure rail washers during the fall.

Davey said that during peak fall weeks the washers will run two to three lines a day.

"We run it between rush hours, probably on weekends but not so much at night because it's loud," he said. "We probably do two to three lines a day."

The tank can hold 22,000 gallons of water, capable of cleaning for seven hours.

Davey said he has seen improvements in slippery rail delays since the washer has been in use.

"In the last three years, slippery rail delays have been reduced by 10 to 40 percent since we put these rail sprayers out in 2005," he said. As the number of commuter rail riders increase because of rising gas prices, the commuter rail system sees the track washer as a way of letting passengers enjoy fall foliage without worrying about time slipping away.

"We'll keep doing it over the fall, so that when train get back up there, the traction should be good," Davey said.