SAN FRANCISCO (CBS SF) — The company that inspected a section of rail between the 16th Street and Civic Station BART stations in San Francisco that broke last week and caused a major service disruption noticed an anomaly in that area last September, a BART executive said Thursday.

Paul Oversier, the transit agency’s assistant general manager of operations, told BART directors that inspectors from Sperry Rail Service of Danbury, Conn., had thought that the anomaly resulted from “slag,” or excess material, where BART had done some welding between the two San Francisco stations.

However, Oversier said BART officials now think that may not have been the case and the anomaly might have resulted from something else, although they don’t yet know what that problem was or whether it contributed to the 10-inch break in the rail.

The break was reported at about 9:20 a.m. on May 6 and wasn’t fixed until about 4 p.m. that day. During the repairs, BART could only run trains on a single track through downtown San Francisco, which led to long delays and big crowds throughout the day.

BART passengers on the platform at Embarcadero station in San Francisco during delays in the system, May 6, 2015. Note the message board instructions. (Nicole Jones/CBS)

BART passengers on the platform at Embarcadero station in San Francisco during delays in the system, May 6, 2015. Note the message board instructions. (Nicole Jones/CBS)

BART officials only learned about the anomaly when they contacted Sperry after the problem last week and asked for a detailed report on the inspection that was conducted last September, Oversier said.

He said, “It was not reported to us as having been a problem” at the time.

Oversier said that in retrospect, BART workers who did the welding probably should have knocked off the excess material or ground it down instead of leaving it hanging, even if the material didn’t contribute to the problem last week, as it’s important to have clean rail.

“It’s an open question” whether the excess material contributed to the problem but it’s not a best practice to leave excess material alone,” he said.

The rail section that broke was installed in 2011 and regular ultrasonic tests had indicated that it was still in good shape, according to Oversier.

He said BART’s investigation into the broken rail is looking into whether there were any problems in the manufacturing process for the rail section that broke and other rail sections that were installed in 2011.

BART is conducting tests of those sections using new technology that’s better at discovering problems that are close to the surface, Oversier said. In the past it has been hard to pick up on such problems, he said.

The testing will be completed by the end of this week, Oversier said.

A second problem also hit BART on May 6 when a power outage caused by a metallic balloon that hit a power line forced the closure of three stations in Alameda County for about 90 minutes in the afternoon.

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