The MAX Blue Line is the original East-West MAX line that brought modern light rail to the Pacific Northwest and was only MAX line in Portland throughout the twentieth century. The first section (later named Eastside MAX when the MAX extension to the west opened) began service on September 5, 1986 between Gresham and Downtown Portland. The second portion of the Blue Line (branded Westside MAX) opened in 1998, extending service to Hatfield in the western suburbs. This project included the building of Portland MAX’s only underground tunnel. The 2.9 Robertson Tunnel is extremely deep and includes MAX’s only underground station at Washington Park. Washington Park is the deepest subway station in North America at 259 feet below the surface of the park above.

For the first few years the Eastside and Westside MAX branding was to separately brand what were always through trains on the only MAX line at the time between Cleveland Avenue in Gresham (what destination signs say) and Hatfield Government Center in Hillsboro (what destination signs say) running through Downtown. In 2001, due to the opening of the Red Line (the second MAX line to open) the line was named the Blue Line.

When the Blue Line opened trains used 26 high-floor Type 1 LRVs manufactured by Bombardier. In 1997 MAX placed into service the first low-floor light rail trains to be delivered in North America with 52 Type 2s (the unique SD600 and SD660s, MAX is the only operator) and later 27 visually similar Type 2s entering service. These trains (and the provision of at least one Type 2 or 3 car always coupled with a Type 1 car) allowed full rider activated accessibility on all trains via a rider-operated button. Small-fixed lifts used to be located at each stop to raise wheelchair passengers onto trains for step-free access. As of 2024 the Type 1s are finally being replaced by more Type 6 (S700) trains that will make all of MAX low-floor. This will also give MAX a fleet entirely built by Siemens that has built all LRVs after the Type 1s.