I think some dogs are just better cut out for it than others. My red-heeler is a saintly trail dog, even though we never trained her to be. I take her to the farm as often as possible, partly because she is fat, and partly because she will protect me and alert me if something is amiss. I did most of her training. Sit, lie down, come, stay, shake, jump, high five. And she naturally keeps a safe distance from horses, most of the time. If she gets too bold, I say "Remy, move" and she does.
My other dog, Oreo, is a lab chow mix, and I trained him too, but not well enough for him to respect me at all times like Remy does. He is very ornery and likes to do his own thing, so while I can take him on trails, I typically don't because be might stray too far and get hurt. We have to cross a road and he doesn't always go across, sometimes he'll just run alongside or even in the middle! He also likes to run off. He comes back, but I still don't like him out of my sight.
Sure, if I had been better about training him (I was only twelve, and I was gone for three months in the summer, so I didn't quite understand the importance of strict obedience training), he would be much more reliable. But he would still have the urge to go off on his own, while Rem naturally sticks with me.
Plus, he is WAY too friendly to guard.
Again, training + the dog's natural disposition play very important roles into what makes a good trail dog.
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EDIT: Oreo does everything that Remy does, except for "come". They also both roll over. Not very useful, but still. Remy enjoys pleasing her humans and Oreo likes a pet/scratch/treat. :)