The J Line offers frequent, all-stops service along the El Monte Busway and the Harbor Transitway, two grade-separated transit facilities built into the Southern California freeway system.

As J Line buses travel along the El Monte Busway and the Harbor Transitway, they serve stations built into the center or side of the roadway.

Buses travel between the western end of the El Monte Busway and the northern end of the Harbor Transitway along 3.6 miles (5.8 km) of surface streets in Downtown Los Angeles where J Line buses make a limited number of stops near major employment centers, tourist destinations and Metro Rail stations.

In San Pedro, Line 950 buses once again travel along surface streets, serving the Harbor Beacon Park & Ride and making frequent stops along Pacific Avenue.

J Line buses run 24 hours a day between El Monte Station, Downtown Los Angeles, and the Harbor Gateway Transit Center, as route 910.

[4] TAP vending machines are available at most stations (except Carson and Pacific Coast Highway) and near most street stops in Downtown Los Angeles.

However, because vending machines are unavailable at all stations and street stops, passengers who need to purchase a card or add funds can do so at the farebox on board the bus.

Metro and Foothill Transit offer a reciprocal fare program in which pass holders may ride either J Line or Silver Streak buses between Downtown Los Angeles and the El Monte Station.

[5] The idea for the route now known as the J Line came in 1993, as Los Angeles County Metropolitan Transportation Authority (Metro) staff studied how to operate buses on the Harbor Transitway, which was under construction and would open three years later in the summer of 1996.

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.

[14] Since the start of the J Line, Metro has been working on refurbishing the aging stations along both the Harbor Transitway and the El Monte Busway.

All the Harbor Transitway stations were refurbished with real-time arrival signs, new wayfinding signage, improved lighting, and soundproofing by late 2012.

Transit Access Pass (TAP) card ticket machines were added to stations in early 2017 to support all-door boarding on J Line buses.

[19] As part of Metro's NextGen Bus Plan, the agency had proposed discontinuing the J Line's route 950, which offers service to San Pedro.

[21] The San Pedro neighborhood opposed the change, with citizens requesting that they also receive electrified bus service and maintain a one-seat ride to Downtown LA and El Monte.

[25] As a result of that investigation, Metro added concrete-filled metal bollards to all stations on the Harbor Transitway and the El Monte Busway to prevent vehicles from entering the platform.