Ths diesel is for the dynamic braking that a steam engine does not have. Dynamic braking is another way of applying the train brakes. With steam when the brakes are applied the air is dropped and the air pulls the unto the wheels, but the dynamic brakes work the same way an automobile brake does. It grabes the rim anx applies pressure slowly until the train stops or fast in case of an emergency. A train going 60 mph would require up to a mile to stop with a consist of 100 or more cars.

The cars behind the 844 steam engine were the 2 Tool Cars Lockman and Nystrom hauling parts for the Bigboy and the Northern in case the are needed, Howard Fogg Power Car second car suppling electric to the cars, Willie James a Sleeper for the crew, Coach for crew, Cheyenne 48 seat Diner, City of San Francisco Dome Lounge Coach, Idaho Observation. These were the cars that left Omaha with the consist. They added extra Coaches and Dome Lounges charging riders $5000.00 for a seat in the Dome Coach and $3,000.00 a seat in Coach seating. Train was sold out on return back to Cheyenne.
The car consist was as follows Art Lockman Tool Car, Howard Fogg Power Car, Lynn Nynstrom Tool Car, Willie James Sleeper, 2037 Coach, Cheyene 48 Seat Diner, City of San Francisco Dome Lounge Coach, Idaho Observation.

Now the diesel makes sense, but your explanation of dynamic brakes doesn't. Dynamic brakes don't work the same way automobile brakes work, at least no automobile I'm familiar with. Dynamic brakes, unlike air brakes, are not mechanical, they are electronic. When the brakes are applied, the traction motors on the diesel locomotive become generators and the electricity is sent to big resistors that basically turn the electricity into heat. The higher the resistance the more braking action. That's why the diesel locomotive has big cooling fans on top, to dissipate the heat from braking.
Although I never worked for the railroad, I grew up in a railroad family and actually know a little about these things. Both my parents, my uncles, my brother and some of my cousins, and my grandfather worked for the PRR then Conrail or SEPTA.
Anyway, it makes sense to have the diesel for additional braking since the big steamers don't have traction motors.