The population of the City of Chicago is 2.7 million.
The population of the Chicago metropolitan area i.e. "Chicagoland" is 9.5 million (almost the size of Michigan). So 6.7 million people are technically not in the city.
A significant fraction of them commute to downtown Chicago for work.
Might be hard to find that, but based on the info I can find, a good many do commute in to the city. You could dig down more into station level numbers to get a more accurate count. For example, if you wanted to exclude stations on the line that are within city limits, but not downtown.
Anecdotally, as someone whose walk to work (from other transit) takes them against the flow of people coming from Union Station: it's a lot of people indeed.
That's unexpectedly low, I'd say. In SZ, the daily commuter count approaches at least 2.5-3 million for people living on municipality's border.
Shenzhen has twice as many people as Chicago. So, if we extrapolate, the amount of people commuting will double for Chicago if Chicago were to be turned into Shenzhen.
That tells that much, much, more people in Chicago work outside of urban centre, or the portion of population who is working is simply less.
