While I was at the top of Mt. Washington last week, and in North Conway, NH as well, one cog train came up mostly empty, and went down the same way, while the other one was basically full. It is fall color season, but not the maximum summer season. Talking to the agent on the top of the mountain, the fact that the railroad is diesel now, has not changed ridership at all, but has dramatically reduced its costs.
North Conway was running several trains each day to Conway, and to Bartlett, as well as one train to Crawford Notch and back. The passenger load was virtually young people with very young children and older people whose children are grown. Again, not peak summer season, but people coming for the fall colors. By my guess, passenger loads ranged around 40-50% of capacity, but the fact that it was all diesel did not seem to bother anyone at all, even though they have had steam in the past.
Trains are trains, scenery is scenery. Scenery beats motive power for 99% of train riders. If you don't have scenery, then you need something other than a train to bring in passengers on a regular basis. You either have to have promotions, like Polar Express, Pumpkin days, Easter Egg hunts or you need to run STEAM for its novelty.