It could be as simple as clarifying what the customer is paying for to avoid complaints.
When making reservations, the customer, for whatever reason, wants the early train - notes it costs less and books the tickets. Low and behold, they show up at the station and a diesel is coupled up to the train.
Now the customer grumbles to whomever they can who works for the D&S. "I paid for a ride on a steam train. Your advertising says steam powered trains. Now I find out no steam on this train? What's with the steam locomotives I see back there? = points to the roundhouse area.
Granted, the ride and the scenery is more important to most riders, but, if the customer expected steam as part of the deal and thinks that's what they paid for, you now have a customer with their smart phone out leaving a low review on Yelp. Part of the ride is the experience of stepping back in time. The rider doesn't know that diesel is a relic in its own right. Steam is what "back in time" means.
We can hope, right?