BART is trying to determine what caused a train to derail Wednesday just south of the Oakland City Center/12th Street Station.

Seventy-five passengers were aboard the Richmond-bound train when three of the four front wheels on the lead car jumped the tracks at 10:24 a.m. about 250 feet from the station, BART said.

Oakland firefighters helped lead the passengers on foot to the 12th Street station. BART spokesman Jim Allison said two people required medical assistance, including a man who suffered a seizure.

Another BART spokesman, Linton Johnson, described the incident as "very minor" and said trains "go very slowly through that area." He said the train was probably going about 15 to 30 mph.

BART had no immediate explanation for why the train jumped the tracks.

James Rice III, 41, of Oakland, said he was riding in the lead car when it began "jumping up and down" as soon as it left the Coliseum Station. It did the same thing after it left the Fruitvale and Lake Merritt stations, Rice said, adding that the doors didn't seem to be working properly.

"It was moving side to side and up and down," Rice said of the train. "I thought it was going to jump the tracks. I was wondering why they didn't stop it at Fruitvale or Coliseum and take us off the train."

Rice said, "I'm not angry, just surprised and shocked." He said he was getting himself checked out at a hospital because his neck was hurting.

The derailment caused delays of 10 to 15 minutes for much of the day on the Fremont-Richmond line, with trains detouring to the West Oakland station around the derailment site.

The derailed car was put back on the track at 1:45 p.m. and taken to BART's Hayward service yard for inspection.