BART service between the Pittsburg-Bay Point and North Concord stations will be shut down indefinitely — and overall service will suffer for months — as the transit agency tries to get to the bottom of a baffling track problem crippling trains by the dozens, officials said Thursday.

Transit officials warned passengers Thursday afternoon that for the foreseeable future, riders who want to travel between the Contra Costa County stations will have to use a bus bridge arranged by the transit agency through four bus agencies. Those buses will travel on freeways that are sure to be more packed as they deliver riders to train cars that are sure to be more crowded.

BART mechanics said Thursday that they were closer to pinning down the problem. They said 50 train cars that failed Wednesday were hit with a power spike as they moved through a track crossover north of the North Concord station. On that stretch of track the power is reaching up to 2,000 volts — twice what BART expects for normal operations.

On each of the broken-down cars, the surge caused a semiconductor device called a thyristor to fail. BART said the parts — which are critical to each car’s propulsion system — cost $1,000 to replace and must be specially manufactured. That will take months, and riders should expect delays and shorter, more crowded trains in the meantime.

BART has 669 total train cars and is supposed to be running 570 at any given time. But it is now down 58 cars.

The problem with the thyristors is only happening on what BART calls “C” cars, which have older equipment than other cars in the fleet. There are about 250 of these cars in BART’s fleet. However, the agency cannot simply remove the “C” cars from service because the power surges have knocked out less-expensive fuses in the other cars as well.

“The silver lining is that this is no longer intermittent. All of the cars are failing in the exact same way,” said Dave Hardt, BART’s chief mechanical officer. “The recovery of the fleet is going to take months. A lot of that is parts, some is elbow grease.”

Earlier trouble near tube

A similar problem occurred in late February in West Oakland when a voltage spike near the Transbay Tube damaged some 80 cars. Hardt said that the same part caused the trouble in West Oakland, but that BART hasn’t figured out exactly why it is happening and why it is occurring now.

In Oakland, BART turned off a nearby electrical substation that had been installed a few months earlier. The situation improved, officials said, but did not go away completely.

The voltage spikes have not endangered riders, BART said.

At the Pittsburg/Bay Point station Thursday morning, a line of BART officials guided commuters to a stream of waiting buses. “Station’s closed,” the crew members said as they pointed riders to the buses. Whenever one bus filled up with commuters, another pulled up to take more.

Riders distressed

BART was waiving parking fees at the station and allowing passengers through the turnstiles for free, but it was unclear if that will continue. BART officials said they will have to reimburse the four agencies lending them buses, but gave no cost estimate.

The situation left some riders fuming.

Clarence Griffin, a 28-year-old security guard who works in San Francisco, said the buses were moving with more efficiency Thursday than on Wednesday night. Griffin said his trip from Pittsburg to San Francisco on Wednesday night took him an hour and a half — double the time it usually takes, nearly making him late for work.

“It sucks. It’s an inconvenience,” he said. “There needs to be more consistency.”

Cass Ramsey, a 32-year-old Lodi resident who works in Berkeley, got to the station around 6 a.m. Thursday, only to continue her commute by car instead of jumping on a bus.

“It’s an inconvenience, but it is what it is,” she said. “There’s nothing you can do about it, so there’s no reason to get mad.”

Emanuel Spikes, a 28-year-old man who waited in line for a bus, said the station closure was making him late to work and his 7-year-old daughter, who was with him, late to school. Still, Spikes said the bus bridge appeared to work pretty well and was better than nothing.

“Hopefully they fix it ASAP,” he said.

As BART works to fix the problem, the agency is gearing up for a series of unrelated weekend shutdowns on the Fremont line starting at the end of the month. Service between the San Leandro and Bay Fair stations will be cut off March 26 and 27, then again April 9 and 10, and on four other weekends through June.

The shutdowns are necessary to replace 2,000 feet of worn tracks, 950 wooden ties and a host of other track components, officials said.

Kale Williams, Hamed Aleaziz and Rachel Swan are San Francisco Chronicle staff writers. Email: kwilliams@sfchronicle.com, haleaziz@sfchronicle.com and rswan@sfchronicle Twitter: @sfkale @Haleaziz @rachelswan