The J Line opened on December 13, 2009, utilizing the pre-existing Harbor Transitway and El Monte Busway, semi-exclusive roadways that are used by both buses and as high-occupancy toll lanes.

Buses on both lines use a silver livery (similar to the one used on Metro Rail), passengers can board at any door, and vehicles receive priority at intersections.

The Los Angeles County Metropolitan Transportation Authority (Metro) acquired the Southern Pacific Burbank Branch right of way in 1991, intending to utilize the corridor for light rail trains.

[4] Neighbors successfully petitioned lawmakers to pass a ban on utilizing transit tax revenue for anything other than an extension of the existing subway into the San Fernando Valley.

With both subway and light rail now legally prohibited, but with growing political pressure to utilize the former railbed, the last available option was a dedicated busway.

The new higher frequency service would be funded by converting both corridors into high occupancy toll (HOT) lanes, to be branded as the Metro ExpressLanes.

[8] On June 30, 2012, the busway used by the G Line buses was extended northward to the Chatsworth Metrolink station, continuing to follow the former Southern Pacific Burbank Branch right-of-way owned by Metro.