With blistering temperatures into the mid-90s today, about one-third of the BART's 669-car fleet could get unusually hot, spokeswoman Luna Salaver said.
"One third of the cars tend to have air conditioning issues when ambient temperatures rise above 90 degrees," she said.
Konstandinos Goumenidis, 28, a student at the Art Institute of California on Market Street, encountered a hot train this week as he traveled from Fremont to San Francisco.
"It was very hot in one of the cars on Tuesday, probably hotter than it was outside," Goumenidis said.
"I moved to the second car so I could hopefully get a cooler car," he said. "About half the car was cool and the other half was not."
While BART's air conditioning systems could use updating, BART spokesman Linton Johnson said, "There are a lot of other competing needs right now."
BART faces a capital funding deficit of about $7.5 billion over the next 30 years. Johnson said that, among other improvements, BART needs to replace its entire rail fleet.
"We do the best we can," spokeswoman Salaver said. She said fixing the temperamental cars would cost about $9 million.
BART riders who find themselves in hot train cars are encouraged to notify the train operator so that one of six roving technicians can address the problem.
Johnson has created a video explaining why some train cars get hot and how riders can report the problem.
BART service near outlying stations, like Dublin/Pleasanton and Pittsburg/Bay Point, is particularly vulnerable to heat-related problems including stalled trains or cars that overheat from older air conditioning systems.
"The farther out you go in the East Bay, the hotter it gets," Johnson said.
Sometimes in hot weather, a network of boxes that controls the train cars malfunctions, Johnson said. The system helps regulate train speed and verify that a train can safely move along segments of track without causing an accident.
If one box overheats, Johnson said, a train could stall or its operator may have to take over manual control, which means it couldn't travel more than 25 mph.
"It's like going down the highway and suddenly having to go from 60 to 10 miles per hour," Johnson said. "Equipment doesn't like heat. There's nothing you can do to stop it from overheating."
Johnson was not aware of any equipment problems caused by the latest heat wave.
"Nothing major has happened in the last few days," he said.
BART and Caltrain both experienced heat-related service delays in August.
Caltrain spokeswoman Christine Dunn explained that when it is this hot out, the rails can expand in the heat and be damaged by trains' weight. Slowing down trains reduces pressure on the rails, she said.
The current heat wave has also not caused any Caltrain problems.