Both incidents came during a period of oppressively hot weather, and Metro spokesman Dan Stessel said high temperatures probably caused three cars on a Green Line train to derail Friday afternoon near the West Hyattsville station.
After baking in 99-degree weather, a portion of the track apparently buckled. It took Prince George’s County rescue workers about an hour to clear the train and lead the 55 passengers through a nearby tunnel and out of a ventilation shaft that serves as an emergency exit. No injuries were reported.
But since announcing Saturday that heat was the probable cause of the derailment, Metro has declined to provide additional details, including when the stretch of track was last inspected.
The Tri-State Oversight Committee, which investigates Metro accidents, is looking into both Green Line incidents. But the committee said Metro, which is typically required to submit preliminary reports within three days, had been granted an extension until Thursday.
Friday’s derailment marked the second time in three months that one of Metro’s trains jumped the track with passengers aboard. In April, an outbound Blue Line train carrying 1,000 passengers derailed at Rosslyn. The cause of that incident was human error — a switch was not property clamped — and an employee involved in the incident was fired, Stessel said.
Derailments occur in many rail systems, and in recent years, some systems, such as those in Boston, Chicago and San Francisco, have experienced two derailments in close succession, as Metro has this year. But whether the timing represents more than a coincidence is difficult to determine.
“It’s hard to say whether it’s a trend,” said Keith Holloway, a spokesman for the National Transportation Safety Board.
According to federal statistics and Metro, between 2008 and the first part of 2012, there were 35 derailments among six of the major U.S. subway systems — Chicago, Boston, Atlanta, New York, San Francisco and Washington. Metro has had six during that period, including the incident Friday. San Francisco and Boston each had five derailments in that time, New York had seven and Chicago had 12.
When measured by 100 million passenger miles — a standard basis of comparison — Boston and Chicago have the highest rate of derailments in recent years among the six systems compared. The San Francisco and Washington systems fall in the middle of the six, followed by New York and Atlanta.
The numbers, which transit authorities report to the federal government, include only derailments that happen during regular service hours and do not take into account incidents that may have happened in rail yards or when trains are out of regular service.