Metro officials said yesterday that four train derailments over the past 18 months, including one at Silver Spring in August, probably were caused by a combination of poor maintenance and problems with a new type of rail car.

Metro officials said all four derailments involved new 5000 series rail cars, occurred on side tracks and involved trains with no passengers aboard operating at speeds of 5 to 10 mph that were making tight turns on tracks that were not properly lubricated.

Officials said such conditions could cause problems with the new rail cars, first used in 2003, because they are lighter than other types and their wheels fit slightly differently on the rails, even though the cars and tracks meet industry standards.

Fred Goodine, Metro's assistant general manager for safety, compared the problems to those faced by sport-utility vehicles that have trouble negotiating highway ramps designed for smaller cars.

Likewise, he said, the 5000 series rail cars "may not be as tolerant to any deviation" from normal operation on Metro's tracks. He cited similar problems with the same type of cars in Pittsburgh and Boston and said a task force would look at possible changes to the cars or tracks.

Goodine said the probability of a derailment of this type on a main line was unlikely, largely because passengers would add weight to the cars.

There were no passengers on the train that popped off a side track as it was making a routine turnaround just north of the Silver Spring stop in August. Officials said yesterday that the train veered 62 feet before stopping, revising earlier statements that it ground to an immediate halt.

Goodine said wear on a switch point was a contributing factor in that incident, as was a lack of lubricant on the rails. Without proper lubricant, increased friction between the wheels and track helped pop the train off course, he said.

Asked why the track lacked the proper lubricant, Goodine said, "In all honesty, that wasn't followed through on." He said all tracks have since been properly lubricated.

Goodine also said human error was ruled out. No one was hurt in the incident -- or any of the other derailments -- though it caused schedule changes and delays along the Red Line because trains could not turn around at Silver Spring for hours.

Goodine recommended a review of standards for acceptable wear on the tracks at switch points, installation of guardrails on switches like the one involved in the derailment, tracking trends in rail inspection reports and implementing a policy for a wheel and rail study whenever a new type of train is used.