Vents Along the Metra Tracks?

Spotted this on my way in to work, and realized… I have no real idea what this is, or what it’s purpose is. My best guess is that it’s serving as some kind of ventilation, as it seems to be located above a street?

I can’t really zoom in, or walk over and peer down… so I figured the best I could do is post a photo and ask around on the Internet. Maybe there’s machinery located underneath the ground here, to control switches and such?

It was just interesting to me that this is something I walk by most every day, but remain clueless as to its actual function. Maybe it’s super obvious and something I’m just missing, but I’m curious.

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The Fix is Not the Solution
MSI Nearby: A Sudden Realization that the Chicago Museum of Science and Industry is Much Closer to Our House Than We Suspected