Built in an era of cutbacks in government funding and attempts to balance the federal budget, the CTA's Orange Line was inexpensively constructed almost entirely using existing railroad rights-of-way. Almost the entire line parallels main line railroads, making it an interesting line for railfans to ride.

Leaving the Roosevelt Road stop south of the Loop, near 15th Street trains cross over tracks of the Illinois Central line going west. This IC line also connects with the St. Charles Air Line. At 17th Street, Orange Line trains rise and turn west over the connector above 18th Street, built in 1969 for the Dan Ryan line. Green Line trains continue south over the original elevated line south of 17th Street. After crossing Metra's Rock Island District, the original connector turns south towards the Dan Ryan segment. Orange Line trains continue west over the new elevated segment constructed especially for this line, and cross over the new connection between the Dan Ryan segment and the State Street Subway. The Orange Line then turns southwest and parallels the IC line crossed previously at 15th Street.

The Orange Line passes over Amtrak's 21st Street interlocking, south of Union Station. Amtrak's Union Station trackage comes from the north and crosses the Chicago River over a vertical lift bridge. South of the river, Norfolk Southern assumes ownership of the ex-PRR line continuing south over the IC line, and the IC ex-GM&O line turns southwest to parallel the original IC line and the CTA Orange Line. Until recently, the two IC lines paralleled each other all the way to Bridgeport. But a connection was recently constructed, feeding the ex-GM&O traffic onto the original IC line. Some of the ex-GM&O trackage has been made into part of the IC's small yard at Bridgeport.

The Orange Line parallels the IC through Halsted Street. The Orange Line occupies right-of-way originally occupied by the Santa Fe Railway, south of the IC. At Bridgeport, the three double track lines of the Santa Fe, IC, and GM&O originally converged onto one double track bridge over the South Branch of the Chicago River, before splitting again west of the bridge. The Orange Line now has its own bridge immediately to the south, east of the Ashland Avenue stop.

West of Bridgeport, the original IC line turns west, and what is left of the Santa Fe line begins, paralleling the IC to the south. The only user of that Santa Fe line had been Amtrak's Southwest Chief. But with the transfer in 1996 of that Amtrak train to the Burlington Northern line in Chicago, that Santa Fe line's days may be numbered. The ex-GM&O line continues southwest, with the Orange Line paralleling to the south, passing the 35th/Archer stop.

West of 35th/Archer, the Orange Line rises, offering a spectacular view of the Brighton Park crossing. This crossing is not interlocked, and all trains must stop before proceding across. The ex-GM&O line crosses the CSX ex-B&O "Panhandle" line. The Orange Line sweeps south, crossing Western Avenue twice before actually heading southeast to parallel the CSX line. CN's Railport Yard is also in the area, to the east. CN owns Grand Trunk Western, whose line passes over the CSX line at 49th Street.

Also at 49th Street, the Orange Line turns west, crossing Western Avenue a third time while stopping there. The Orange Line parallels the CN line to the north. West of the Kedzie Avenue stop, the Orange Line rises to pass over a junction between CN, Santa Fe, and the Indiana Harbor Belt. CN turns south near the location of its old Elston Yard, Santa Fe is to the north with its Corwith Yard visible from the Orange Line, and the IHB line begins there and proceeds west at ground level.

The Orange Line parallels the IHB line on a new elevated structure to the north, past the Pulaski stop, before turning south near Kenton Avenue. The Orange Line passes over both the IHB and the Belt Railway of Chicago, which cross each other at BRC's 55th Street interlocking. 55th Street is also the junction with the BRC line which turns west to reach its Clearing Yard from the west end. The Orange Line parallels the BRC main line to the west the rest of the way into Midway Airport.