But a car starts at the minute the occupants want to start, from the place they want to start, and goes as close as possible to where they want to be. A trip by rail (or bus) needs to factor in the frequency of the service (and its reliability) and places it will pick up and drop off. The comparison of a car drive from the centre of Ballater, or Banchory, to the centre of Aberdeen with a train trip is largely irrelevant unless the user can work to the timetable and wants to leave and arrive very close to the stations.
The prospect of sufficient likely volumes to justify a fraction of the expense seems unlikely, even to Banchory.
The success of the Border Railway is due to it operating through a more populous region (Galashiels 16,000, Banchory under 8,000) and the ability to achieve some quite high speeds. Nevertheless it received massive public support and will require further support as long as it runs. Fare income from a few 2 car trains that might possibly be busy, and a lot more that are almost empty, won't make a business case. It takes a huge weight of other factors to overcome that.
It would probably be cheaper to offer potential Ballater line users a free taxi service for life. It would certainly be more flexible for most of them