The way I train trail horses is... Well, I trail ride. I take a green horse out on the trails (with another horse for the first few rides, and then alone), and I ride him like I would ride any old trail horse. We cross all the creeks, pick through the vines, step over the logs. If I need to clean trails or put up flagging, that horse better learn that job as we go along. If I need to sidepass/turn on the haunches/position the horse, we learn it on the trails in real life situations. So many people baby their green trail horses. Think they "aren't ready" for real trail riding. But if you wait until they're "ready," you'll be waiting a long time.
I also don't let horses "look" at spooky things. I just don't make a big deal out of it. We ride by it. We might ride by it side ways or backwards
but I ignore it. No big deal. Maybe it's a weird way to go about it, or the "wrong way", but all my horses are bombproof after a few weeks.
I also do a lot of groundwork -- halter and lead rope, controlling the feet. Lunging, backing, sidepassing, turns on forehand and haunches, moving on foot at a time in any direction.
People think the weirdest thing I do is teach my horses to be "trustworthy" -- able to be ridden without micromanagement. Give them responsibility. I teach them to ride on a loose rein in all gaits. I take the bridles off and ride all gaits in an arena with no regard for direction, just continued forward motion. Keep in a trot without trying to walk or canter. Stuff like that. Stop with no reins. Continue in a straight line without me having to hold them between the reins.