In downtown Chicago, the Green Line operates over the famous Union Loop 'L' structure along with Brown Line (Ravenswood), Orange Line (Midway), Pink Line (Douglas) and Purple Line Express (weekday rush hours only) trains. East of Clinton Street, the route bridges Metra's Union Pacific railroad tracks (which terminate just south of here at Ogilvie Transportation Center) and the Chicago River before joining the Union Loop 'L' tracks at Wells Street. The 'L' bridges a couple of railroad tracks (the Belt Railway of Chicago's main line and Union Pacific's Rockwell Subdivision) before entering downtown Chicago at Clinton Street. Here, the Green Line tracks diverge from the railroad embankment and continue east on a steel elevated structure directly above Lake Street, a major east–west thoroughfare.

Beginning at the yard and inspection facilities in Forest Park, the Green Line runs east through Oak Park towards the city on an embankment adjacent to Metra's Union Pacific/West Line tracks from the Harlem Avenue terminal, on the border of Oak Park and Forest Park, to a point just west of Laramie Avenue.

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