You've got it backwards. They hardly ever use a diesel, and then only for very specific reasons.

"Back in the day," UP's steam passenger trains were around 12-14 cars; and they still used diesels in places like the Blue Mountains. Today, 18-25 cars is the closer to the norm, and they have run as many as 34.

Still, unless they are facing some long, hard grades (either up or down), such as Cajon, Donner, the FRC, the Blues, Cima, King, Reverse, etc., there won't be any diesel there, unless they are ferrying one or more of the Heritage Units (the E9's or the 6936) to an event that will feature both steam and diesel. And when they use diesels on the grades listed, they usually add them close to the beginning of the hills and take them off as soon as they are past the hills. For example, on the 844 West Coast trip earlier this year, they used a diesel to Ogden, and from Sparks to Roseville. The diesel was hauled dead from Ogden to Elko where it was set out. On the return trip, they used a diesel from Roseville to Portola (hauling it dead from Roseville to Oroville). The diesel was left at Portola, and it was steam only from there to Ogden.

The genset they hauled from Roseville to Oakland and over Altamont Pass back to Roseville was for display; it was never even started up.

The diesel on the Frontier Days train is there because the grade from Carr to Speer is too much for 1 engine with 24 cars (in steam days, this was helper territory - helpers were added at Carr) and because there is intense pressure to get the passengers to Cheyenne as quickly as possible. No doubt, 3985 could handle a 25 car train alone on that grade, but it would be a much longer trip in terms of time. The people who ride that train could not care less about crawling up that grade with the stack roaring; they want to get to the rodeo NOW!

Last year, 3985 drug the 6936 from Cheyenne to North Platte for display, left it there, and went on to Iowa and Minnesota and back to Cheyenne without any diesel. So out of around 2,000 miles total on that trip, there was a diesel on 225 miles, and it wasn't doing anything but riding along.

There are railroads that insist that a diesel accompany any steam locomotive anywhere it goes. UP is not one of them. This is not to be confused with using an HEP-equipped diesel along with a steam locomotive so the passengers can have such things as heat in the winter, AC in the summer, and lights and working toilets and water systems year round.