Feasibility studies conducted in the early 1990s shifted plans away from I-205 and culminated in the South/North Corridor project, which failed to secure voter-backed funding over several ballot measures.

On November 3, 2020, voters declined a tax ballot measure that would have provided local funding and put the project on hold.

[6] A panel of local and state officials known as the Joint Policy Advisory Committee on Transportation (JPACT) endorsed the I-205 route in 1987 with a request to start preliminary engineering for light rail along this corridor in lieu of an originally planned busway.

[7][8] Their preferred alignment had been the I-205 busway, a partially completed, grade-separated transit right-of-way built during I-205's construction several years prior.

[11][12] In an effort to settle the dispute, Metro updated its regional transportation plan (RTP) in January 1989 to reassert the westside line's priority and commission preliminary work for the I-205 and McLoughlin Boulevard proposals.

[13] The U.S. Senate Committee on Appropriations approved a financing package later in September, which provided $2 million to assess the two segments, but at the behest of U.S.

[14][15] Alignment studies initially examined extending the proposed I-205 route further north across the Columbia River to Vancouver Mall or the Clark County Fairgrounds.

[17] A 25-mile (40 km) route from Hazel Dell, Washington through downtown Portland to Clackamas Town Center called the "South/North Corridor" was finalized in 1994.

[18] Estimated to cost around $2.8 billion, Portland area voters approved a $475 million bond measure in November 1994 to cover Oregon's share.

[19] A Clark County vote to fund Washington's portion, which would have been sourced through sales and vehicle excise tax increases, was subsequently defeated on February 7, 1995.

[20] TriMet later sought funding for various scaled-back revisions of the South/North project following a general route between North Portland and Clackamas Town Center that voters went on to reject in 1996 and 1998.

[33]: P-2  TriMet published the combined "I-205/Portland Mall" final environmental impact statement in November 2004 and began acquiring land in 2005.

[46] In February 2004, TriMet awarded the I-205 segment's design–build contract to South Corridor Constructors, a joint venture between Stacy and Witbeck, F.E.

[58] To address its $31 million budget deficit caused by the slow growth of payroll tax revenue amid the Great Recession,[64][65] TriMet simultaneously eliminated four bus lines and implemented service cuts to 49 other routes.

[69][70] Following the South/North project's cancellation, the city held off revitalization efforts for the transit mall amid proposals from local businesses to rebuild it to allow curbside parking.

[71] In 2003, TriMet planners reconsidered adding light rail to the Portland Transit Mall after planning for the second phase of the South Corridor Transportation Study, or the Portland–Milwaukie line, revealed that a fourth service on the existing tracks in downtown's Morrison and Yamhill streets—already served by the Blue, Red, and upcoming Yellow lines—would exceed that segment's capacity.

[26][28][73] TriMet conducted a study proposing stations on either the left, right, or middle lanes of the transit mall and ultimately selected a hybrid center-lane travel with right-side boarding option in April 2004.

[80][81] Owing to techniques learned from the Interstate MAX project, businesses were kept open while sections, from north to south, were closed off in three- to four-block increments.

[88] TriMet began line testing in January 2009, initially with light rail cars hauled by a truck,[89] then with its new Type 4 MAX trains.

[90] On August 30, the 1.8-mile-long (2.9 km) Portland Mall light rail segment opened with inaugural service from the Yellow Line, which TriMet rerouted from First Avenue and Morrison and Yamhill streets.

[93] The extension would have connected riders to the Marquam Hill campus of Oregon Health & Science University (OHSU) with an inclined elevator and to Portland Community College (PCC) Sylvania with a shuttle bus.

[94] A new Hall Boulevard station would have connected with WES via Tigard Transit Center and would have served as the site of a new operations and maintenance facility.

[94]: 170–175 Metro adopted its 2035 RTP in June 2010 where it identified a segment of OR 99W between Portland and Sherwood as the region's next highest-priority "high-capacity transit" corridor.

[96]: 1  In June 2013, the project steering committee selected light rail and bus rapid transit as the alternatives for further consideration.

[101] The following year, proposals to serve Marquam Hill and Hillsdale with tunnels were dropped from the plan because they would be too costly, have severe construction impacts, and attract few new transit riders.

[102][103] In May 2016, the steering committee voted to select light rail as the preferred mode alternative over bus rapid transit.

[109][110] In light of a budget gap of $462 million, planners proposed reducing lanes on Barbur Boulevard and shortening the line's route to terminate in downtown Tigard.

[117] The line continues east onto the Northwest Glisan Street Ramp where the tracks join the Eastside MAX alignment and then cross the Willamette River via the Steel Bridge.

[121] Beyond Gateway Transit Center, the Green Line proceeds south, entering the I-205 MAX extension just east of I-205.

[131] In September 2019, the Green Line was the third-busiest MAX service with an average weekday ridership of 19,160,[132] 1,480 fewer riders than the previous year.