That was very cool :-) Thanks !
I wonder if that diesel #4015 a few cars back was helping at all or just along for the ride ?
The diesel a few cars back accompanies every Union Pacific steam locomotive as a "back-up" in case a locomotive's water supply runs low. It's bad for a locomotive tank to run dry, and so if the water runs low, the diesel takes over. Tanks are replenished from small-town water reserves, after which the diesel is shut down and the steam locomotive takes over again. The diesel is never used to help the steam locomotive, which doesn't need help anyway, and which pulls the non-running diesel along with the rest of the train.
The Union Pacific operates this 4-8-8-4 and a 4-8-4 (which was never retired from active service) and after restoring its 4-6-6-4, donated it to a railway operation in Illinois.