I got the chance today to spy IC 3140 running light through Mundelein on the CN tracks. It ran up to the north-side, switched tracks and ran south. I was on foot with a little weiner dog, so I couldn’t chase. It is the first time I have seen an engine run light outside of a yard.
What I think you saw was the power a local getting itself on the right track to do some work.
When I worked in Deerfield I’d go over the either the CN or CP (Metra) on my lunch hour. An IC unit working the local on the old WC/Soo/WC/CN was kind of comon.