The grade separation sure is interesting! Must have been a former mainline on one of those tracks!
This is a very cool bridge to visit and offers an amazing view. The bridge crosses the railroad at the crest of a grade and sits very high above the tracks. It's easily one of the highest points in the area and can be seen for miles.
The reason for the marked difference in the tracks is that the Illinois Central railroad wasn't happy with the grade and came through in the 1960s and cut the one track way down. It was the original siding, but was made the main after they cut it down.