TriMet and municipalities along the proposed route subsequently produced a modified alignment that would have connected the Clackamas Town Center and the Kenton neighborhood in north Portland.

Certain parts of this alignment, including trackage through downtown Milwaukie and Portland, generated significant controversy. Funding measures went to the polls statewide in 1996 and in the Portland metro area in 1998, losing in close votes each time.

Despite these defeats, TriMet continued to project an alignment between northern Clackamas County and Clark County, Wash. A modification to this routing, announced March 16, 1999, extended light rail north from the Rose Quarter Transit Center to the Multnomah County Expo Center along the Columbia River.

The Metro council, the City of Portland and the TriMet board all subsequently approved the project. Construction began Feb. 19, 2001. Twenty-seven Siemens SD660 low-floor cars, numbered in the 300 series, joined the light rail fleet in anticipation of this service. These began arriving at the Ruby Junction shops in February 2003 and entering revenue service in September 2003 after initial testing. The line opened May 1, 2004.

The line opened Sept. 10, 2001, serving stations along the I-205 corridor north of Gateway and in the industrial and commercial core east of the airport. It terminates at the south end of the main terminal building, adjacent to Concourse A. TriMet added six SD600 cars to the fleet for this service. The route was subsequently extended to the Beaverton Transit Center. 

Clark County, Wash. and Clackamas County took a fresh look at light rail, so TriMet planners began discussions on up to four possible extensions, two into Clackamas County and two into Vancouver, Wash:

• From Gateway Transit Center south to Clackamas Town Center, following the I-205 alignment.
• From downtown Portland to Milwaukie, crossing the Willamette River and following an alignment along the Union Pacific railroad tracks and McLoughlin Boulevard.
• From the Portland Expo Center north across the Columbia River into Vancouver, then north-northeast to Vancouver Mall, following I-5 and State Highway 500. This would extend the Interstate Avenue alignment.
• From Gateway Transit Center north to Vancouver Mall following the I-205 alignment. This would tie in with the existing airport line.

TriMet and local government officials announced Feb. 17, 2003 that they had selected a plan to construct both the I-205 and Milwaukie alignments. The Clackamas route would come first, combining the right-of-way along I-205 with the existing east-west line from Gateway Transit Center to downtown Portland.

The project also included a new alignment in downtown Portland, using the Transit Mall on SW 5th and 6th Avenues. This route serves both the Union Station area and the Portland State University campus. Construction began in January 2007, and the line opened Sep. 12, 2009.

Siemens built 22 new S70 low-floor light rail cars, numbered 401-422, to increase the TriMet fleet for the new service. The first cars arrived in June 2008 and entered service in August 2009.

The next new route, the Orange Line from downtown Portland to Milwaukie, opened in September 2015. It crosses the Willamette River on a transit-only bridge between the South Waterfront area and OMSI, used by MAX trains, Portland Streetcar, TriMet buses and pedestrians.

The route includes an at-grade crossing of the Oregon Pacific RR east of OMSI, and parallels the Union Pacific RR and the Portland & Western RR through southeast Portland into Milwaukie. It terminates south of downtown Milwaukie in the Oak Grove neighborhood. The agency acquired 18 new S70 cars, numbered 521-538, for the fleet.

TriMet is currently planning for a route through southwest Portland to serve Tigard and Tualatin. Preliminary reports indicate that the route would follow SW Barbur Boulevard from Portland to Tigard, then follow the Portland & Western, ex-SP Tillamook Branch right-of-way to the Bridgeport Village shopping and entertainment complex in Tualatin.

TriMet has contracted with Siemens for new SD700 cars, based on the S70 design, to replace its 1986-vintage Bombardier cars and increase the rolling stock fleet for the anticipated southwest route. The first 26 would replace the 26 100-series Bombardier LRCs, which are approaching 35 years in service. The first of these new 600-series cars arrived in December 2022.