OAKLAND (CBS SF) – A breakdown during an overnight test of BART's 'Train Of The Future' triggered a major delay on the transit system's East Bay lines early Friday morning.

BART officials said a problem developed with the train before 5 a.m. near or inside the Lake Merritt station.

BART spokesman Jim Allison said the train was returning from a test run when a problem developed.

"It was a four-car train of new cars returning from testing overnight when it became disabled," he told KPIX 5. "It's been pushed to a pocket track near the Bay Fair station."

Trains on the Fremont and Dublin-Pleasanton lines were forced to single track while the disabled train was towed to a maintenance facility.

Initially, East Bay commuters experienced delays of 20-30 minutes.

Sixty new cars were expected to hit the rails by the end of the year, 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.