When ever the train runs near an interstate you will have cell service. There are area in Nevada, Utah, and Colorado where the tracks are far enough away from the interstate that there is no cell service. Time sensitive communication would be stressful. Just checking in periodically would not be a problem.
My guess would be only about 20% of time without service. It will, of course, vary from carrier to carrier. I think Verizon has the most coverage in the west. Don't think that a cell booster would help much.
The signal boosters that some work trucks use would probably help a lot except for the fact that you don't have a practical method to put the antenna outside. Verizon used to sell Jetpacks with antennas that could be affixed to windows for improved service but I haven't see those in years. I usually get intermittent service that works okay near towns and highways but falls off in remote areas. As said above it's enough to check-in or take a quick call but not enough to work remotely.
I tell anyone likely to need to get in touch with me on long-distance trains to use text messaging. In turn I'll send text messages and they'll be picked up along the way. I'm old enough to have sent and received Western Union telegrams so it's easy for me but it's actually easier for a lot of purposes than being in a conversation that suddenly drops off when a mountain gets in the way.
That said, eastbound about half an hour after leaving the Moffat Tunnel you should have good signals until about 45 minutes east of Denver. Some carrier may do better.