BART officials are investigating what caused a year-old part that helps connect train cars to snap suddenly Tuesday morning, causing the nine-car train to come apart before stopping at the west edge of the Transbay Tube, snarling the commute.

"A metal bar that isn't supposed to break, that's supposed to last a lifetime, broke somehow," said Linton Johnson, a BART spokesman.

BART officials rushed the broken piece to a laboratory for a metallurgical analysis. But until a cause is determined, they have no plans to inspect similar devices - perhaps as many as 1,168 - on the transit agency's 669-car fleet.

"There's no reason to believe we have a safety problem," said Paul Oversier, BART's deputy general manager for operations. "And if we were to inspect right now, we wouldn't know what to look for."

The yoke assembly, a metal part of a mechanical device that couples rail cars, broke at about 6:23 a.m. as a train traveling from Pittsburg/Baypoint to San Francisco International Airport was nearing the west end of the Transbay Tube and about to enter Embarcadero Station. A safety system automatically applied the brakes on both segments of the train, bringing them to a stop about 70 feet apart.

The train operator brought the first section of the train - five cars - into Embarcadero Station at about 6:55 a.m. Another train operator was brought in to drive the remaining four-car segment into the station at about 7:10 a.m. BART used the eastbound track to run trains in both directions until it could inspect the tracks and restore full service at about 7:25 a.m. Delays of up to 40 minutes continued throughout the morning commute.

"It was not at all good for customers," Oversier said of the delays.

Fractured part

Both segments of the train were taken to the Daly City yard and inspected. Mechanics found that the yolk assembly, a round solid-steel piece about 4 inches in diameter and 12 inches long, had fractured. The piece, which sits inside the coupling device, was installed in January 2009 as part of a years-long routine maintenance effort in which BART is gradually replacing yoke assemblies. Each car has two of the devices, and BART mechanics have replaced them on about 85 percent of the fleet.

In addition to the metallurgical examination, BART is also working with the manufacturer, Wabtec Corp, of Wilmerding, Pa. Johnson said BART has never had a similar problem and does not believe any other transit agencies have either. Wabtec officials could not be reached for comment.

Yoke assemblies are subjected to a huge amount of stress, both when cars are coupled and during normal operations, Oversier said. Because BART has so many of the parts in service, and never had a problem until Tuesday, he said he feels confident that the agency can wait for the laboratory results. He said he doesn't know how long the lab tests would take.

"But this is a priority for us," he said.

Single-tracking

Tuesday's uncoupling, which Johnson described as "pretty bizarre," happened at just about the worst time and place. There are roughly 175,000 Transbay trips each weekday, and during commute hours, BART runs trains through the 3.6-mile tube about every two minutes.

Only one track runs in each direction through Transbay Tube, and crossover tracks - which allow trains to switch tracks or change directions - are located at West Oakland and an area between Embarcadero and Powell stations.

That makes it impossible for BART to handle commute traffic on a single track, and it led to significant delays with trains waiting in line to go through the tube.

It is not uncommon for BART to briefly run trains on a single track - a practice known as single-tracking - when a train becomes disabled in the Transbay Tube. But Oversier said that extended single-tracking, as on Tuesday, is fairly rare, occurring perhaps once every two or three years. Complete shutdowns of the tube are even less common, he said, occurring about once a decade.