The Yellow Line and Orange Line share a homepage because they are operated as one combined line with nearly all trains through-running between the lines. Southbound trains become Orange Line trains as they cross the Steel Bridge and enter the Portland Transit Mall, northbound trains make the change just before the PSU South/Southwest 6th and College Stations. This means signs in the Portland transit mall display different route bullets depending upon the direction of service at the station. Northbound stops are for Green and Yellow Lines (just like when the mall opened), Southbound stops are Green and Orange Lines.
Historically the Yellow Line, Interstate MAX was the first part of this line to open. A new 5.8 mile long north extension largely following Interstate Blvd opened on May 1, 2004. This line was originally designed to cross the Columbia River (along a new bridge combined with the rebuilding of the falling apart I-5 Interstate Bridge) into Vancouver, Washington but politics have continued to get in the way with trains terminating at Expo Center in View of the River. At the southern end trains crossed the Steel bridge and originally ran into Downtown on the only alignment in service when the line opened following the Red and Blue Lines, before terminating at the Blue Lines original turning loop between the Library and Galleria Stations in Downtown Portland.
Trains in 2009 were relocated with the opening of the Green Line along the new light rail tracks that run north-south along the Portland Transit Mall.
The Orange Line (Portland-Milwaukie MAX) opened on September 12, 2015 as a new southern extension from the Portland Transit Mall. The Orange Line name had been heavily used in branding throughout the construction process of the Portland-Milwaukie MAX so for branding purposes it made sense to brand trains along this Branch as the Orange Line, even though it leads to trains changing their destinations mid-route by interlining the Yellow and Orange Lines. This 7.3 mile long extension included the opening of a new bridge over the Willamette River via Tilikum Crossing which is used exclusively by pedestrians, cyclists, MAX trains, streetcars (sharing the tracks but no stations directly with MAX) and some bus routes. No private vehicles are allowed on this slender cable-stained bridge.