I realize that no horse is totally "bomb proof", and that goes for my Alax, who has spooked with the best of them. However, in reading back posts on this thread, if a horse can't handle a pool noodle, a piece of PVC, or a broomstick attached to their side, I would NOT drive it. As much as Alax has spooked at goats, black asphalt "holes", water crossings, etc., he had to be totally OK with whatever we "threw" at him in the training process before I would put him to. If they flip out over going backwards with the shaft trainers attached, it's time to back up in the training process. I have used both shaft trainers and the cart the initial put to, but before we get to that step, we have "played with lots of toys". We have a old rug, plastic bag, pieces of foam, bubble wrap, playground ball, pipe insulation, plastic drain tube tile, and my personal favorite (until my kid destroyed it) shiny silver expandable tube leftover from our skylight installation. We use things that squeak and pop, whips that crack (you can't control what some other idiot does next to your horse), etc. The whole point is to expose them to lots of different stuff that they realize will not hurt them. We do it in a "matter of fact" sort of way, "this is your job now to carry the plastic bag on your head from point A to point B". Once they realize that the easy thing is OK, we add stuff that is "harder", but all the time encouraging them to be a "big boy/girl". We DON'T come at it with the perspective of, "It's OK, I know it is scary, but don't spook." That is just telling them that there must be something to be afraid of.
And in reference to Alax spooking nowadays, he has come a LONG way since we have applied the "Mom and Dad is the boss about everything, and horses are bad managers" philosophy. He gladly gives up the decision making process, and trusts us for the most part that we aren't going to make him do something that will hurt him. Last night, when I drove him through the woods, there was a tree down in the trail. I decided to have him go around it which included driving over small branches and some brush. The cart wheels and his feet over the sticks and leaves were so loud, that I really had to raise my voice so that he could hear me tell him, "You can do it." The branches scraped on the bottom of the cart, but he plowed through like a draft horse! I was impressed. When he was done, we trotted off like it was nothing. I still wouldn't call him bomb proof, but this is a horse that used to freak out over the flick of the brush in the grooming process! We are in the process of training our coming two year old Arab/DHH filly. She, too, has to "do her good work" and carry the bag on her head while walking over the plastic tube. Whatever you can find around the barn will work. And yes, I also believe that there are some horses that will never drive.