Regarding the Jacobite 'Harry Potter' train that runs between Fort William and Mallaig, crossing the Glenfinnan viaduct (as seen in the films)...
As an avid railway photographer, I would gladly phot a steam excursion from lineside, but as a participant of a railtour, it really does not matter what the motive power is.
Fortunately there are enough people who do think it's worth paying to travel behind steam or there'd be no steam to photograph! Ideally the trip should be enhanced by a welcoming boarding experience (a chance for a leisurely inspection of the haulage, including in this case during the mid-point attachment perhaps?) as well as that onboard experience and being made to feel appreciated for keeping the show on the (iron) road.
We included this recently in a 60th birthday trip. The trip was over 12 days, and we were very lucky in that it coincided with 2 weeks of fantastic weather - the guide on our 3 day tour of the Outer Hebrides said that it was the first time in 13 years that it hadn't rained once during the tour.
However, it also meant that they didn't run the steam train on the Jacobite due to the risk of setting fire to the surrounding very dry countryside - we didn't find that out until we got to the station. Plus we'd also been warned beforehand that the usual posh carriages were undergoing some sort of maintenance I think, so we ended up in just what seemed like old (1950s?) and somewhat tired rolling stock.
Given that we'd paid £105 each (1st class), plus £30 each for the 'high tea' (in a box), it was a bit disappointing, although we still enjoyed it - the glorious weather did mean that the scenery looked fantastic.
I suspect the passengers probably weren't the only ones disappointed - the hillsides at both ends of the viaduct were covered in people either taking photos or just watching, presumably hoping for a bit of steam rather than fumes
https://westcoastrailways.co.uk/jacobite/steam-train-trip