There is an elevator on the sidewalk at the southwest corner of Michigan and Van Buren. This takes you down to a tunnel underneath Grant Park, to the Van Buren St. Metra station.

Here's where it is on a map (the red square, not the Google Maps pin):

And here's what it looks like in Google street-view:

Search terms: Chicago, Van Buren station, Metra, ME line, wheelchair-accessible, wheelchair access, ADA-accessible entrance

(I'm posting this because I had a hard time finding information about this online. There are a few different ways to access the Van Buren Metra station, and I wanted to know ahead of time how to enter the station without having to go up or down any stairs.)