I LOVE that locomotive, I've been all over it, in the cab, and seen it run several times. I wish they would repaint it back to the two tone gray scheme, but it was too expensive and difficult to maintain. Did you know that UP 844 (or 8444) was never retired from the active roster list? And that makes the Union Pacific the only Class 1 railroad to never completely dieselize.
Most of the time when diesels are used in conjuction with steam engines it is for braking requirements only, as mandated by the railroad it's running on. Steam engines don't have dynamic brakes.