One of BART’s new train cars overshot the end of the track and ran into a mound of dirt at a Hayward testing facility on Friday — another setback for a transit agency that has been dealing with aging infrastructure and a mysterious track problem crippling trains.

Officials were investigating whether an operator mistake or system error caused the glitch, which occurred at 1:55 p.m. on a test track between South Hayward Station and Union City, said Taylor Huckaby, a BART spokesman.

The train was traveling on a straight track and continued going after the track ended, causing some of the train to remain on the track while the rest went into the dirt, he said.

Only the operator was inside the train car at the time, and the train was traveling less than 10 mph, Huckaby said. There were no injuries and officials said the train was not significantly damaged.

A new operator was being trained on the system, part of BART’s incoming fleet of 775 cars that could start carrying passengers by the end of the year.

Sixty new cars are expected to hit the rails in 2017, with the full order of 775 expected by 2021. BART is spending $2.5 billion to replace and expand its aging fleet of 664 cars. The agency hopes to find the funding to expand that order to 1,081 cars.

BART mechanics have yet to determine the cause of a track problem that stopped service between the Pittsburg-Bay Point and North Concord stations last month. A power surge disabled dozens of cars and forced passengers riding between the stations to use a bus bridge. Normal train service at the stations resumed April 2.

Jenna Lyons is a San Francisco Chronicle staff writer. Email: jlyons@sfchronicle.com Twitter: @JennaJourno