I visited the area briefly in 2019, so take my info for what you will.
At the time, the Deseret Western was running I THINK 4 days per week. One of the days mid week-either tues/weds/or Thurs, was a "work" /Track day where no trains ran. On the other days, sometimes they would run two trains- one in the morning and a second in the afternoon. I can't recall if there was a pattern or not on which days the second train normally ran. I want to say the info I found on trainorders ? was mostly accurate.
The morning train was the one I caught - Unfortunately it's against the sun for the most part, as they start at the mine, head west to the powerplant, and then return - and by the time they return it's close ish to noon. It's not the easiest line to chase, since they kinda go Northeast to Southwest directionally, whereas the roads in the area are like a drunken grid being mostly east /west (US Hwy 40) and North south (Colorado State Highway 64, Utah State highway 45). The tracks are also by and large grade separated and the trains aren't exactly crawling along.
That said, despite how unappealing that may sound, with Black Mesa and Lake Powell having ceased in 2019, I am pretty sure Deseret Western is the last of it's kind (electric catenary powered mine to powerplant) left in the US or Canada. So for me, it was, and remains, something absolutely worth seeing.
If you go out there, it's SUPER sparsely populated, so make sure gas tank is full AND make sure you have water- more than you think you'll need. The sun, especially at elevation with little tree cover, is no joke.
Despite the challenges, it's possible, but tricky, to get multiple shots- I'd show up near the mine in the morning around daybreak and just wait.
https://maps.app.goo.gl/uDq3CPpX39qPNUqM7 Puts you near the mine.
From there, once you get them moving, go up to US 40 and hang a left to start heading east. You'll see the tracks for at least part of that initial stretch. You might be able to get a shot at Blue mountain road, which is the next decent sized road, but personally, I'd head straight for Dinosaur, and make a left (Southbound) onto Colorado Hwy 64. From there, Before you cross the tracks, you can hang a left on what I think is county highway 21/stanton road, that'll take you to Hwy 45/Bonanza, and from there hang a right, and head back north till you cross over the tracks. The overpass here, and the embankments on either side, are, IIRC, your last shot of the tracks before they get onto powerplant property, which is just west of here.
Look at the line carefully on googlemaps, or earth, or whatever mapping platform you want to use- but be careful, as the terrain is either above or below the track level- rarely at grade, and the catenary can make decent shots tricky, along with the actual sunlight itself. Also watch out, as again the area is super rural, and only a few of the roads in the area besides the state highways are paved, and google could be (and often is) a bit off on some of these less marked roads. Some may be washed out as well.
It's a beautiful (and lonely) stretch of incredibly unique railroading and would be a must see/do for me, but I get that it's not for everyone.
If you get out there, catch the morning train, and are happy with that - you have a few options at your disposal.
I had enough time to make it back to Promontory and visit the golden spike historical park before they closed, though it was a bit tight. From a pure historical aspect, it was interesting- just to see the site itself and get the lay of the land. Probably better for a more "casual" interest (the National park needs to try and appeal to as many as possible), but might be worth checking out.
The area around the small town of Dinsosaur itself has some interesting hiking/parks nearby- the area has a unique beauty too it for sure. So if you need a break from trains..
Going south- you can take Colorado hwy 64 to Rangely, pickup 139, and head south towards Grand Junction and the Ex Rio Grande Mainline.
Take Hwy 40 east, and you'll wind up in Craig, on what I believe was the aptly named Craig Branch of the Rio Grande, and I think still in place/used by UP ? Not sure how much/how often though.
Hwy 40 East will either take you almost all the way back to SLC/Provo (and by the Heber Valley railroad before either of those two), or you can pickup US 191 in Duschene and head towards Helper. Watch your speed in/around the towns, they are often speed traps.
All in all, if you want to hit the Deseret and Western, you should be able to hit the morning train, and hopefully get at least SOMETHING else in daylight, either on the UP overland route closer to Ogden, or on the ex Rio Grande Main. For me though, I'd burn a day, possibly two, in hopes of getting the Deseret Western.
As far as other stuff in the area railroad wise- not sure how visible /accessible it is, but one operation I wish I'd spent time on out there was the Kennecott /Rio Tinto smelter/copper mining area between Magna and Tooele. At one time, they had these unique cab'd Geeps. When I was out there, they still had at least one, but I only saw it in passing from the highway, and I was in a hurry to head further west.
Be careful if you go out there in the summer. I did go in July, and it gets HOT. Bring more water than you need. Stop and get gas more often than you think, especially if you veer off the beaten path.
If you proceed west into Nevada- 80 (which roughly follows the ex WP till a little past Wendover) also goes right by the Bonneville salt flats which is right near the Utah/Nevada border- a worthy spot to stop for a minute and think about the famous speed records that have been set over there.
If you've already gone after the Deseret Western, Ely Nevada and the Nevada Northern really isn't THAT much further from the greater Salt Lake City area- do able as a LONG day trip if you start early enough. This is another spot that sadly I can't really comment on, as my travels have yet to take me there, but they run a good operation.
All in all, if I was going for pure train counts and western scenery, I'd probably pick Cajon Pass or somewhere along either the Sunset Route or the ex Santa Fe Transcon vs going to Utah. You could fly into LA, rent a car, and spend a couple days hitting up Cajon, Tehachapi, and part of both the SP and Santa Fe Coastline routes (assuming the later isn't washed out) all in a long weekend. That said, I'd rather get one or two trains along an area I want to see even somewhere with less traffic, and especially with less people traffic. "Out West" can mean many different things, and each area has it's own charm and beauty, and all of which is worth seeing.