TriMet, more formally known as the Tri-County Metropolitan Transportation District of Oregon, is a public agency that operates mass transit in a region that spans most of the Portland metropolitan area in the U.S. state of Oregon.

Created in 1969 by the Oregon legislature, the district replaced five private bus companies that operated in the three counties; Multnomah, Washington, and Clackamas.

TriMet started operating a light rail system named MAX in 1986, which has since been expanded to 5 lines that now cover, as well as a commuter rail line in 2009.

It also provides the operators and maintenance personnel for the City of Portland-owned Portland Streetcar system.

Portland is a port and the largest city in the U.S. state of Oregon and the seat of Multnomah County.

It is in the Willamette Valley region of the Pacific Northwest, at the confluence of the Willamette and Columbia Rivers.

The city covers and had an estimated population of 632,309 in 2015, making it the 26th most populous city in the United States.

Approximately 2,389,228 people live in the Portland metropolitan statistical area (MSA), the 23rd most populous MSA in the United States.

Its Combined Statistical Area (CSA) ranks 17th with a population of 3,022,178.

Roughly 60% of Oregon's population resides within the Portland metropolitan area.

Light rail, light rail transit (LRT) or fast tram is urban public transport using rolling stock similar to a tramway, but operating at a higher capacity, and often on an exclusive right-of-way.

Commuter rail, also called suburban rail, is a passenger rail transport service that primarily operates between a city centre, and the middle to outer suburbs beyond 15 km (10 miles) and commuter towns or other locations that draw large numbers of commuters—people who travel on a daily basis.

Trains operate following a schedule, at speeds varying from 50 to 200 km/h (30 to 125 mph).

Distance charges or zone pricing may be used.

Air pollution occurs when harmful substances including particulates and biological molecules are introduced into Earth's atmosphere.

It may cause diseases, allergies or death in humans; it may also cause harm to other living organisms such as animals and food crops, and may damage the natural or built environment.

Human activity and natural processes can both generate air pollution.