I am with GTD, this is not a good idea for the horse trained as a CW racing/driving horse. Their system of driving training is NOT at all similar to what is done with most single driving horses, progressing thru steps of learning, in most cases. Much more similar to Draft horse training, with “hitch him up, let’s get going, the other horses will hold him down” kind of thinking. They need to get him along, see if he is worth taking the time to develop for the CW Teams, before putting more polish on him. He may have been forced to drive, despite fighting, with a large Draft Training Horse hauling him along, regardless of TB horse desires. For a truly fast horse, the owner will “work with him” a bit more to be able to use the speed in his Team.
This is probably not their only CW driving prospect, so whatever mix of training systems they use, there probably is not much in ground work, single horse work, used. Your riding horse has NO CLUE how to go by himself or in a CART!! Hitching him to the cart will PROBABLY get someone hurt or injured, horse or person.
Driving horses in Multiples, Pairs or Fours, with no single work has a big hole in his horse education to try using him alone. He has ALWAYS had a friend beside him in a Pair or Team, not facing scary stuff alone, can bump off his friend to know limits in his ‘space’ for turns, working. Single? He doesn’t have a clue where he should be, shafts have no give, how to behave and respond, since “he never did THIS before!”
Pregnant, you should not be thinking that training him to go Single is a good idea. You are dealing with changing body, your parts don’t always work now as they have in the past. Balance and responses are affected, getting even more restrictive as you are further along. Not a slam, just want you SAFE while working with your horses. Pregnancy changes EVERYTHING. You can always try
working with him later if you are determined on this, but I don’t think this older horse is a good prospect with his previous history of CW racing. He has too much OLD training from the CW racing he has done, so is more likely to respond automatically with that in driving situation, than any new learning he gets. Not Good for either of you when hitched.