The Westside Express Service (WES) is a commuter rail line in the U.S. state of Oregon serving parts of Washington and Clackamas counties in the Portland metropolitan area. Owned by TriMet and operated by Portland & Western Railroad (P&W), the line is 14.7 miles (23.7 km) long and travels north–south from Beaverton to Wilsonville along a route just west of Oregon Highway 217 and Interstate 5 (I-5). WES consists of five stations and connects with MAX Light Rail at Beaverton Transit Center. Service operates on a 45-minute headway on weekdays during the morning and evening rush hours. In Spring 2022, WES saw a daily ridership of 420 passengers or about 109,000 riders annually.

Washington County is one of 36 counties in the U.S. state of Oregon and part of the Portland metropolitan area. The 2020 census recorded the population as 600,372, making it the second most populous county in the state and most populous "Washington County" in the United States. Hillsboro is the county seat and largest city, while other major cities include Beaverton, Tigard, Cornelius, Banks, Gaston, Sherwood, North Plains, and Forest Grove, the county's oldest city. Originally named Twality when created in 1843, the Oregon Territorial Legislature renamed it for the nation's first president in 1849 and included the entire northwest corner of Oregon before new counties were created in 1854. The Tualatin River and its drainage basin lie almost entirely within the county, which shares its boundaries with the Tualatin Valley. It is bordered on the west and north by the Northern Oregon Coast Range, on the south by the Chehalem Mountains, and on the north and east by the Tualatin Mountains, or West Hills.