NorthShore
Conductor
My answer would be a qualified yes. Though commuter waiting seating is limited in both north and south concourse atriums, there is some. I'd say a bit more on the north side. You can see some of the benches in one of my photos. Whether you'd want to remain there for an extended period of time with luggage is another matter. So, definitely drop it off first in the sleeper lounge.
As others have noted, what you'll see is occasional trains come and go, along with their passengers. You might also have to hear the sometimes oddly incessant automated voice track announcers.
There may be better places to railfan, of course, including north of Ogilvie near Canal & Fulton (a GREAT place where the trains north out of Union Station are at street level and Metra trains from Ogilvie at a viaduct overpass); the Halsted Ave bridge a couple blocks south of the Grand Blue Line station (where westbound trains on tracks run under and north/northwest trains from Ogilvie pass on a nearby viaduct), or at street level in the Fulton Market district, anywhere in the blocks just west of Halsted (where it's essentially the same train traffic.). Really, all three of these spots you'll see the same trains, just from different perspectives. You could easily grab lunch somewhere in the latter area. As one can at historic Manny's Deli (or Maxwell Street hot hog/Polish sausage stands) if one opts for the aforementioned (by others) Roosevelt Rd. bridge.
However, unfortunately, none of this is extremely well suited with mobility issues. So, assuming you don't have luggage to drag along, and with due consideration of how limited your walking may be, I might make this suggestion (or, at least, it's a thought.) Take a bus or Uber/cab/whatever you prefer to Millennium Park, a few blocks away. Specifically, have a little stroll through the Lurie Garden (across the street from the Monroe entrance to the Art Institute), if you're up for it. It isn't huge, and much can actually be experienced just by walking in and being present. There are some benches (and planter ledges) where you can sit. Immediately across the street (next to the museum) on Monroe, you'll see (and hear) over the bridge into the path where Metra Electric and South Shore trains pass. (A favorite place of mine, with sights and sounds I consider refreshing and peaceful.) You might even grab lunch at the park cafe or a sandwich place across Michigan Ave on your way back to Union Station, if you wish. A pleasant way to enjoy and take in the "city in a garden" with sights and sounds of trains, nearby, on a nice little layover.
) I did have to roll my big wheelie duffel to the baggage counter at CUS, but they offered to drive it over there. No checked luggage any longer on the LSL from Boston, or I would have checked it through to SAS and not dealt with it at all. There was plenty of Red Cap assistance available. I don't know 
