Don't know about lately, but Mount Washington Cog RR was down to
one steam trip a day. Diesel for the rest because of the environmentalists.
I did see that awhile back they were experimenting with used French fry oil
to make steam. Suspect that came to naught. Or are they still trying?
The cog runs 1 steam trip a day primarily because it's cheaper and faster to run the diesel. A trip on the steam engine used to take 3 hours. When the diesels came along they cut that to 2 hours. They can also run larger cars with the diesel so more buts in the seats.
I had heard that running all steam they were at worst breaking even. Switching to diesel they started making money consistently. The environmentalists being happier was a side benefit.
They've been working on running 2 steam trips a day, but they usually have to be the first (which is the normal trip of the day) and one of the last so that they don't block too many of the much faster diesel trains.
The oil fired experiment happened on one of the engines they are still running today. They were having issues with the focused fire from the burner design that was being tried scorching the crown sheet from how hot it needed to burn to keep up steam. It got converted back to coal after a short while. It's an interesting engine, though, it has a lot of improvements that have been tried all over the years like a feedwater heater.