In downtown SLC, Trax runs in the street entirely, but that's just downtown. Once out of downtown, it uses the route of the former Union Pacific Provo subdivision, completely away from streets, allowing for fast operating speeds.
BTW, Trax is currently being extended from the Jazz arena to the Intermodal Hub (buses, Trax, Frontrunner commuter rail). The street construction techniques are very interesting. I'll get some pictures of the construction and post them on my website.