Grand Canyon Railway - steam is back!

[nachoman]

Any particular reason why the steamer was pulling a diesel unit behind the tender?

Several reasons:
1) Head end power for the air conditioners in the cars.
2) Power to get up the 2.5% grade. The 2-8-2 is good for about 7 cars on its own. During the summer, the train would regularly have 12-14 cars. Considering the railroad never had more than 2 operable steam engines at one time, and one was usually under repair or a boiler wash, it just made more sense to use a diesel helper. Plus, it saved having to use two crews.
3) There is no water along the line, and none at the Grand Canyon. The railroad kept a tank car at the rim in order to refill the tender for the return trip. Using two steamers would have meant twice as much water to refill, and twice as much water to haul to the Grand Canyon. In fact, there were local complains about the railroad using water from the city of williams. The town's water infrastructure was very poor, and during one particularly dry year, the town nearly ran out of water. The town has since drilled a couple of deep (3000 feet deep) wells, and their water situation is better.
4) Possibly for dynamic braking. I am not sure if the ALCos have dynamic brakes, but I know the F40s do.