Not sure if you know it or not, but a few counties in Utah contain almost all of the population. Salt Lake, Utah, Davis are something like 70%. Adjacent counties (Wasatch, Tooele) are maybe another 5%, and Washington County (St.George, Cedar City) are the other 20%.
FrontRunner now links Salt Lake, Davis, and Utah. So 3/4 of the population is in a county with FrontRunner. The share within a mile of FrontRunner is likely even larger. Ideally, we can use FrontRunner as transit spine for the whole region, with TRAX/MAX lines branching off at all the stations.