Ive been a pretty regular customer to Garelochead (and occasionally to other stations) my impression is that there is the occasional berth customer to Dumbarton or Helensburgh but its very rare for the other stations up to Arrocher to have any customers other than walkers taking a trip up the line. I think by far the majority of people are travelling to Fort William. Ive not often still been in the lounge at Crewe but I get the impression there are sometimes maybe half a dozen passengers there mostly during the peak summer season.
I have known the occasional berth customer to Dalmuir, but it’s fairly rare. Helensburgh is quite a common set-down point for passengers in berths, as are Crianlarich, Rannoch and my stop Spean Bridge (also Corrour in the spring, summer and early autumn for hill walkers/when it’s stalking season). The other stops seldom see berth customers in my experience. My impression from the times I’ve used it is that about two thirds of the berth customers are travelling all the way to Fort William.
On the Inverness section, by the way, the intermediate stops between Perth and Inverness are particularly well patronised- as much as half the train embarks/disembarks at stops between Dunkeld and Aviemore, with Pitlochry, Kingussie and Aviemore being particularly popular in both directions. I find that on the Highlander, many of the intermediate stops are just as important as the final destination, and that this ought to be taken to account at times of disruption. I’ve never used the Aberdeen section, but I can imagine that the same is true (particularly of Leuchars, Dundee and Stonehaven).
