...on top of which cruise ships disgorge an enormous number of people quite infrequently and very erratically. There are already problems in S Queensferry due to ships anchoring in the Firth and sending passengers ashore in tenders: All parking is banned in the town to accommodate the coaches needed and the local area gets a complete vacuum in visitors for the whole day as those who land are whisked off into Edinburgh.The vast majority of cruise passengers are generally doing prebooked shore excursions to places like Urquhart Castle, Culloden, Eilean Donan Castle* and Fort William. They require a very high degree of reliability in being able to return folk to the ship at the right time. Neither of these characteristics make the railway a particularly suitable medium for transporting cruise passengers...
In a railway context there is almost no spare rolling stock anyway, and on top of that in the more remote parts of the network we have reduced the infrastructure to the point where we probably couldn't run more trains anyway. I suppose that if DRS were running "their" freight on a line they could juggle the days run so that a tourist train could use the same path, but the timings would probably not fit.