The only illustrations I've found show the horse in the middle of the track, and hitched to the coupling hook, which surprised me, because I thought the holes in GWR solebars were to hook on horse harness. I would think in your situation the horse probably would walk between the rails, which in turn suggests that the track would need to be ballasted at least level with the sleepers, if not right over them. I would have thought the investment in a capstan system would be a much bigger and more sophisticated yard than you seem to have in mind.