Well historically this was the Chicago & Northwestern Main line so there were passenger trains for him at some point. Plus a lot of these crews also count their time in freight service towards their number as well. It is why when someone retires from our HUN Base we see the Amtrak Logo, CSX usually, and whatever predecessor they worked for as well.
The UP West Line trains used to terminate in Genevia before they were extended to Elburn in 2006, that has seen continuous commuter rail service first as the Chicago & Northwestern Commuter Service before the UP bought them out in the mid 90s.
Presumably, he might have worked on any of the historic C&NW lines to Kenosha, Harvard, or Geneva; though it seems clear that his passengers know him from UP/CNW West going back many years. So that's likely the run he's been pulling on seniority for some time.