A cracked rail on a stretch of Metro’s Blue Line near Arlington Cemetery disabled five trains during Friday’s morning commuter rush, causing long delays — in some cases an hour or more — for thousands of passengers, the transit agency said.

Adding to the severity of the problem, Metro said it was forced to suspend inbound Blue Line service from Virginia for nearly two hours, starting at 8 a.m., while track inspectors walked between the Pentagon and Rosslyn stations, trying to locate the cracked rail.

The crack was repaired and normal service resumed at 11:25 a.m., Metro said.

The crack, on the northbound Blue Line just north of the Arlington Cemetery station, caused damage to five trains that rolled over it between 6 a.m. and 8 a.m., Metro said. The damage was not immediately evident, though, and the trains did not break down until later, after each had crossed the Potomac River and reached the District.

Not long after 6 a.m., two trains that had passed over the cracked rail became disabled, one near the Foggy Bottom station, the other near Eastern Market, Metro spokesman Dan Stessel said. Shortly before and just after 7 a.m., he said, two more trains broke down, at the Farragut West and McPherson Square stations.

A fifth Blue Line train from Virginia also broke down in the District, Stessel said, but it was not immediately clear where it became disabled.

With multiple disabled trains struck on eastbound Blue Line tracks in the city, waiting to be towed, trains operating in both directions were forced to take turns using a single stretch of track in several locations at the same time. This caused long delays for Blue Line passengers headed west from Largo and for riders headed east on the Orange, Blue and Silver Lines, which share tracks from Rosslyn through the heart of the city.

The trouble started shortly after 6 a.m. when two Blue Line trains from Virginia broke down about the same time, near Foggy Bottom and Eastern Market. Stessel said both trains were headed to Largo without passengers to start their day of service.

Metro began single-tracking at those locations, waiting for maintenance workers to haul the disabled trains out of the way and determine what was wrong with them.

“Then, in the 7 o’clock hour, there were two, possibly three Blue Line trains that were carrying passengers that also encountered problems,” Stessel said.

He said that five trains becoming disabled in such a short time “points to an infrastructure problem,” meaning the trouble most likely did not originate with the trains but had been caused by a stretch of faulty rail that each of the trains had passed over.

“Working deductively, we were able to make some assumptions about the location of the problem,” Stessel said. For example, because no trains on the Orange or Silver lines had broken down, he said, inspectors concluded that the problem was somewhere on inbound tracks that are used only by Blue Line trains — meaning the inbound tracks between the Franconia-Springfield and Rosslyn stations.

Yellow Line trains also use those tracks, between the Franconia-Springfield and Pentagon stations, before crossing the Potomac on the Yellow Line bridge. Because no Yellow Line trains had become disabled, inspectors determined that the faulty rail had to be somewhere on the inbound tracks between Pentagon and Rosslyn.

By this time, shortly before 8 a.m., the single-tracking in the city had caused delays of a half-hour or more for passengers on the Blue, Orange and Silver lines. Inspectors began walking the tracks from Pentagon to Rossyln, looking for the problem. Meanwhile, to prevent more trains from passing over the faulty rail and later becoming disabled, Metro suspended all inbound Blue Line service, Stessel said.

This caused already long delays on the Blue Line to continue, although they became less severe.

All trains leaving Franconia-Springfield were designated Yellow Line trains and traveled only as far north as the Pentagon station before heading into the city by way of Metro’s Yellow Line bridge over the Potomac. As for riders waiting at the Arlington Cemetery station, shuttle buses began arriving to take them to Foggy Bottom.

Shortly after 9:30 a.m., Stessel said, inspectors found the problem: Just north of the Arlington Cemetery station, he said, there was a crack in the inbound third rail, which carries the electricity that powers the trains.

Subway trains have what are called “collector shoes” protruding from their sides. These are metal paddles that glide along the third rail, collecting electricity. Each subway train has several collectors, Stessel said. A major defect in the third rail might cause all the paddles to snap off, immediately disabling a train. A less severe defect might cause a few of the paddles to snap off, causing the train to break down minutes later.

The latter scenario is what happened Friday morning, with some trains that rolled over the cracked rail breaking down later, in the city, and others continuing to function.

Once the problem had been located, Stessel said, Metro lifted the suspension of inbound Blue Line service at 9:50 a.m. By then, the disabled trains in the city had been towed away, meaning that single-tracking there had halted and delays had begun to ease.

But repairing the cracked third rail meant more single-tracking, this time between the Arlington Cemetery and Foggy Bottom stations. While Orange and Silver line service gradually returned to normal, delays continued for Blue Line riders until the cracked rail had been repaired. Stessel said single-tracking ended at 11:25 a.m.

He said cracks in the Metro system’s third rails occur far less often than problems with the non-electrified tracks, which bear the full weight of trains. He said Friday that inspectors had not yet determined what caused the Blue Line crack.

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