This is a very good question. Between Salt Lake City and Ogden a major issue was capacity. Often, UP's mainline is a parking lot, so a separate, mostly single track (though there are some crossovers to UP for emergencies) Frontrunner line was necessary. Withing SLC the question becomes more complicated. The fact is, UTA has adopted their own standards for track construction (both Frontrunner and TRAX) including concrete ties and crushed stone ballast, as well as extensive work to head off groundwater problems. Within SLC some seemingly good UP track (the old passenger mains on the Frontrunner route and the old Provo subdivision for TRAX) were ripped out, allowing UTA to dig deep, sometimes 6 ft down to reballast and redirect groundwater, even though these factors didn't seem to bother UP. Hope this in part answers your question.
Utah's interurban network is being rebuilt-who'd a thunked it?!
www.utahtransit.us