Reckon with all the foot traffic on the ground looking for him in that area he'd have been spotted...well I'd hope so anyway. The rail-track driver report is a worry for me, said he hit a dog although it might have been a sheep or deer, this must be a common occurance for them and he'd know what was on the tracks. I know you've checked the line up and down but what about actually getting on the train and checking the line up and down from the window view high up for any signs. Bird activity in a localised area is not a good sign so look out for that.
Trying to be more positive what about camping out in the area and cooking up a few sausages and burgers to create a smell that might bring him in about.
If he's got a homing instinct then look at searching an area between your home and Drumochter.
You'll probably have been in touch with them but what about all the contractors in the area working on the Pylons have they seen anything, their dotted around in some of the hotel and B&B accomodations in the area.
Has anyone got any mates in the Police dog handler squad that could help out on their days off to try and pick up a scent or as said already any MRT looking to "train" in the area.
At this time no idea is a bad idea.....Merry-walker(Jac) stay positive but look after yourself lass.