Up on the grand canyon railway, the summer trains nearly always have a diesel behind the steamer. The grades on that line are steep, and even the 2-8-2 can only handle 7-8 passenger cars. Their business is great enough that the trains are usually longer than that. Plus, many cars have electric appliances (air conditioning) that need a generator. If a diesel with HEP is not on the train, they must pull a generator car. And also, they must carry tank cars for water to the grand canyon to water the steamer for the return trip. They have never had more than two steamers operable at one time - and doubleheaders are rare. It is basically an issue of cost. If they doublehead steamers every day, they probably wouln't be running any at all. In the cab of each steamer is a diesel control stand. Yes, the grand canyon's steamers can be MUed with the diesels, and only one crew is needed.
My brother works for the grand canyon railway, and he says they do get occasional complaints about a diesel being on the train. Usually the complainers quiet down once they are described the situation. I have ridden that train probably near 30 times, and I believe most passengers have no idea what the difference between a steamer and a diesel is. I once heard a conversation between two women: "Is there a steam or diesel engine on the train?" "I don't know, what's the difference?" "A steam engine is one of those old fashioned looking engines."
The steamers are also not there just for "show". Every engine on the train is needed for the grades. More than once, one of the alco FP4's will shut down, and the train cannot make it up the hill. A rescue engine will have to be dispatched to help it the rest of the way. I rememeber once the 2-8-2's throttle got stuck while at the canyon, and the engine could not move. They had to send an extra engine to the canyon just to get the train home.
The reality is, on the grand canyon railway, they probably would not lose much ridership if they dropped the steam program altogether. But eliminating the steamers would greatly reduce costs. If it wasn't for the fact the owner happens to like steam, there would be none running in arizona at all.
kevin