From Transit.Wiki


eBART, officially known as BART to Antioch, refers to the eastern segment of the Antioch - SFO line between Antioch and Pittsburg/Bay Point. This segment uses standard-gauge diesel multiple unit trains rather than wide-gauge electric trains so they're not compatible with the main BART system. A transfer platform is built just east of the main Pittsburg/Bay Point platform to permit timed cross platform transfers between both trains.

The reason to implement diesel trains on the eastern segment is the significant cost reduction compared to building a regular BART line, in a low-density environment where long subway-like trains are not justified.