The Purple Line of the Chicago "L" is a 3.9-mile (6.3 km) route on the northernmost section of the system. The service begins from Linden station in Wilmette, north of Chicago, and ends at Howard station in the Rogers Park neighborhood on Chicago's North Side, along the way making various stops at stations in the city of Evanston.

During weekday morning and afternoon rush hours, the Purple Line extends another 10.3 miles (16.6 km) south on the North Side Main Line, making its entire run in rush hour from Howard to Downtown Chicago. During these periods, the line runs express from Howard to Wilson, then express from Wilson to Belmont, then making all local stops from Belmont to the Loop. The express service is known as the Purple Line Express (or the Evanston Express).

In 2024, the average weekday boardings on the Purple Line was 4,904. It is the shortest route in the CTA rail system except during weekday peaks and rush hours.

Prior to the color-coding of CTA rail lines in 1993, the Purple Line was known as the Evanston Line, Evanston Service or Evanston Shuttle, and the Purple Line Express was called the Evanston Express.

The Purple Line is useful for reaching Northwestern University (Foster and Noyes stops in Evanston), including the sports facilities Ryan Field, Rocky Miller Park, Welsh-Ryan Arena, and Canal Shores Golf Course — all at the Central stop — and the Bahá'í House of Worship at Linden. The selection of purple as the line's color was likely from Northwestern's official school color.

Beginning at Linden Avenue in Wilmette, which contains a small storage yard and car service shop, the Purple Line traverses the North Shore area on private right-of-way which begins at street grade. Running southeasterly from Wilmette, the line rises past Isabella Street on the Wilmette-Evanston border, then bridges the North Shore Channel immediately north of Central Street, the first stop in Evanston. The line, now on an elevated embankment, curves southward parallel to Sherman Avenue. Continuing south, the line enters downtown Evanston and stops at Davis Street, then curves southeasterly again to parallel Chicago Avenue and Metra's Union Pacific Railroad right-of-way to just north of the CTA's Howard Yard facilities.

Here the line crosses through the yard area before the junction with the Red Line and the Yellow Line. The tracks are split on grade-separated structures to allow Yellow Line trains to enter the junction from the west. Immediately south of the yard lies the Howard Street terminal, where Red, Purple and Yellow Line trains all terminate.

There are four operational tracks starting at Howard Street, continuing southward to Armitage. During weekday rush hours, the Purple Line runs express on the outer tracks, skipping all stops until Wilson Avenue, and then to Belmont Avenue. From Belmont Avenue to Armitage Avenue, the Purple and Brown Lines share the outside tracks and both make all stops along the route. From just south of Armitage Avenue to just north of Chicago Avenue, they are on a two-track line on a 4-track structure. After the North Shore Line ceased operations in 1963, the outer tracks in this area were rarely used and received next to no maintenance; they were permanently taken out of service in 1976.