CONCORD — BART officials now believe metallic dust is to blame for the mysterious power surges that knocked out nearly two dozen train cars in the East Bay Friday, and two similar incidents last year.

BART workers first started noticing problems with the cars at around 10:30 a.m. Friday between the North Concord/Martinez and Pittsburg/Bay Point stations, said spokesman Jim Allison. The agency ultimately switched to shuttle trains at 1:30 p.m., after the power surges had disabled 22 train cars. They used the shuttle train until about 9:30 that night when normal service resumed, he said.

Allison didn’t have an exact figure Monday for the number of disabled trains that have since been repaired, but he said the car shortage is putting the agency in a bit of a bind. BART had 546 train cars in service Monday afternoon, but Allison said it typically aims to have about 595 cars running at any given time.

“It means that some trains are going to be a car short,” he said. “It doesn’t equate to 49 trains (595 minus 546) because built into the 595 standard is a number that can be swapped out for bad cars during the day.”

It’s not the first time that voltage spikes disabled BART cars. The agency dealt with similar problems last year — first in late February near the West Oakland station, and again in mid-March between the North Concord and Pittsburg stations. The mysterious power surges befuddled BART engineers, who spent weeks trying to identify a solution to the issue that appeared and then disappeared with little warning.

As of June last year, BART had spent approximately $2.24 million to diagnose the problem, including nearly $201,000 in consultant fees and more than $771,000 on new testing equipment. Allison said he could not immediately provide an update on the total cost spent diagnosing the problem since then. He also said he would not be able to provide a copy on Monday of the consultants’ final report to BART, but he said BART workers were able to figure out problem and solution on their own.

The power issues afflicting the 22 cars on Friday and last year appear to the be the result of a combination of factors, Allison said. One issue is the buildup of metallic dust on the underside of cars impacted by the electrical surging, he said.

BART generates metallic dust from grinding the rails, a practice the agency uses to reduce the screeching noise of wheels grating against the tracks, Allison said. The dust can conduct electricity, which becomes susceptible to flashes as the cars draw power from the track’s third rail.

BART engineers noticed the dust was thickest near tunnels and inside the Transbay Tube, where rain and wind cannot naturally wash it away. So, in response to the surging issues near the West Oakland station last year, crews started vacuuming the tracks after grinding the rails to remove some of that dust, Allison said.

The agency hadn’t been as concerned with the dust at its above-ground stations, he said, because the dust is removed naturally by rain and wind. But over the course of the last month, BART workers had been diligently grinding the rails four nights a week between the North Concord and Pittsburg stations.

That stretch of tracks is a little different than the rest of the BART system because it’s on a hill, meaning the trains need a little extra juice to make it up and over, Allison said.

“This particular section of track was designed to provide more power to trains than anywhere else on the BART system,” Allison said. “It is now thought that this rail grinding led to a buildup of metallic dust on rail cars.”

To address the issue, Allison said BART is in the process of purchasing magnets, which will sit on train cars that aren’t carrying passengers to pick up the metallic dust left in the rail grinders’ wake, a solution that Allison would be implemented as soon as next month.