I often cross a railroad track on foot, at this specific point — 41.875593, -87.816695.
From what I've read, this is trespassing because most track in the US is owned by some freight company. It's considered company property, and I guess there are right of way laws that apply to it.
This particular track appears to be owned by CSX. (wikipedia) It appears on their CSX System Map. The name given for this section of track is the "Altenheim Subdivision".
This part of the track has apparently been retired by CSX, since it duplicates nearby track. [2] [3] [4]