During weekday rush hours, the Purple Line extends 10.3 miles south from Howard to downtown Chicago running express from Howard to Belmont, with a single stop at Wilson, and then making all local stops to the Loop. This express service is known as the Purple Line Express. Running a train “express”—not stopping at stations it normally would stop at—is a way to alleviate congestion following a delay.
Adapted from https://en.wikipedia.org/wiki/Purple_Line_(CTA)