With the exception of the loop itself and the lake street el (which run directly over public streets), most of chicago's elevated trackage runs down alleys (where ROWs were easier to acquire by the private companies that built them back in the olden days), so the space underneath is most often utilized for parking.
There's also a lot of el and commuter track that's elevated on earthen embankments, so that space is too impractical/expensive to utilize. Perhaps if chicago was under a severe land crunch we'd see some of that space carved out, but chicago is NOT tokyo (radical understatement).
That said, there are some exceptions (particularly around stations) where you can find the space under the tracks being used by commercial establishments. One of my favorites is the outdoor dining patio of a taco restaurant in our old neighborhood in Edgewater. Our kids (and me too) always got a kick out of eating our tacos while red line el trains rumbled by directly overhead.
Source:
https://www.vanilla-bean.com/localit...e12adb7222cc6#