Just wanted to check some words with you. Martingale for me, is the strap between his front legs, usually to hold the breastcollar down in work. Adjustment shown in demo picture, is very loose to me, so martingale is only a decoration strap as shown. A buxton to me, is a long-shanked curb bit, with a bar at the bottom of shanks. Used mostly in driving multiples, Pairs and Fours. Not seeing it on the photo.
Horse knowing whoa is GREAT, the best gait of a driving horse! If he stops when told, you will both be very happy. Second best gait is stop-and-stand-still for long periods of time. Driver can do a lot with a horse who knows both these skills, is willing to do them when asked. My two favorites.
Be careful out in the field, this is where I said the cart with air-filled tires is not as hardy in use, as the favored wooden wheeled ones. Standardbred jog bike is tougher than the bicycle wheeled cheapies, but still designed for use on groomed track or arena. Very easy to run over things that will flatten the tires, or they break the seal, lose air. If you put sideways torque on the rims, bending them and folding the rim over can happen. Sometimes the whole wheel comes out of the holder when bounced on rough pasture ruts. And sometimes the driver never has a problem. One of those unpredictable things, happens often enough to be a problem we all see. Many folks have walked home after setting out in the air-filled tire carts.
For preparation to hitch, you should practice him dragging things, like tires. Tires have no sharp edges, pull fairly straight, just add drag, not heavy. I never suggest pulling logs or poles, they snag, roll and hop if caught on stuff. Not a good object for teaching safely. Ground driving or long lining and dragging teaches him to move into harness with weight behind him. Gets him brave so a load isn’t scary. Drive him with and without the blinker bridle. He should see and know what is happening behind and around him. Should be a big nothing to him. Blinkers are not to prevent him being frightened. If he acts scared, go back and review his lessons, he missed or doesn’t understand something. Then go to blinkers all the time after putting cart on. Blinkers are to prevent him reading your body language, anticipating commands. He is NEVER allowed to choose what you do while driving, very dangerous.
Get thru all this while you are on the ground, he is totally accepting of everything, before preparing to hitch. This is not a 10 day or 60 day set of lessons. Horse needs to totally understand all you do, ask, respond correctly each time before advancing another step in training. Needs to be comfortable with harness moving on him at all speeds, load pulling and jerking behind him, doing the commands you ask instantly. Then he is probably ready to hitch quietly. Take it easy the first couple times, very easy to overdo with quiet horse. Back to building his confidence in new setting, with cart. Short sessions let him be successful, then think about it when done. Use your watch. Driving is not like riding, no sore rear ends so people often do way too much in an outing. Tired animal is not learning.
The above teach horse his basics. He then has knowledge to use in new or scary situations. Stand still is ALWAYS a good thing, while he absorbs the new information, tries to make it match what he knows so he can respond correctly. Ours are praised for stopping, standing still, while chaos goes on around them. Someone can go to their head for comfort, lead them off if needed, they are GOOD HORSES for being quiet, standing when frightened. Good basics save the day in rough times. All that practice builds horse confidence in you, trusting you will save him from hurt or danger. The horse hitched and driven in a week, doesn’t have the basics knowledge or skills to respond well to new things, he gets scared out there ahead of vehicle all alone. Runaways happen fast, usually within two jumps. You grab control back or things are lost. He doesn’t know you well enough to listen or trust you will take good care of him in dangerous settings. Then he takes charge to save himself.
You might like going back in the archives here, there are many threads on teaching horses to drive. Each is a little different, much good information to be found. Good horse education is crucial, along with a good mind in the animal. Your horse sounds like you have a nice one to start with, trustworthy, they are sure easier to live with over the long haul.