Crystal Lake is a city in McHenry County, Illinois, United States. Named after a lake 1.6 miles (2.6 km) southwest of the city's downtown, Crystal Lake is 45 miles (72 km) northwest of Chicago. The population was 40,269 as of the 2020 census. Crystal Lake is the most populous city in McHenry County, part of the Chicago metropolitan area.

The Union Pacific Northwest Line (UP-NW) is a commuter rail line provided by Metra and operated by the Union Pacific Railroad in Chicago, Illinois and its surrounding suburbs. While Metra does not refer to any of its lines by colors, the timetable accents for the Union Pacific Northwest Line are bright "Viking Yellow," honoring the Chicago & North Western Railway's Viking passenger train.