The difference in Draft work and Carriage work, is that carriage drivers DO NOT want the horses pulling with their front ends unless they are TRULY stuck in heavy going.
Having a horse who PULLS with his front legs is UNDESIRABLE because he is then less able/likely, to be able to lighten/elevate his front end as desired in the jobs carriage driving asks of him. You can’t have light, floaty action, elevation, extension in his Dressage work, speed over the miles, if he is clawing with the front end. Pulling with the front end builds the wrong muscle structures. I do believe any driving horse should be willing, know HOW to pull in heavy ground with his front end if asked. I would not be asking often, but it is a trained-in skill, like handling a rope with a stock working horse. Pulling heavy things is something horse should have done a couple times so an immobile load does not frighten him on the first pull. You ALWAYS hear stories about the horse quitting or rearing, who never pulled a sleigh before. Sleighs may only weigh 300#, but their runners will quickly freeze in snow when stopped. Then sleigh load is often very resistant to a pull if not “broken loose” before asking horse to “walk-on” with the sleigh. The trained horse will stop, listen to direction to pull harder or break loose his runners with sideways movement, THEN walk off with no excitement shown. Runners will freeze fast in cold weather, often only a few minutes so driven horse has to LISTEN well.
Carriage Driving needs a horse using his rear end, since the power/engine of horse starts from behind, ALWAYS. CDE is a relatively new use for light horses, nothing like any previous driving needs. Our forebearers AVOIDED doing much cross-country driving, really hard on vehicles that are not metal!
We didn’t have the time and people able to engage in CDE as a sport until more recent times. Horses were for needed work, rich mens sports. People worked many more hours a week to get a basic living, not much play time. Average folks didn’t own horses, they rented them from the Livery. Pleasure Driving was popular, common, as an enjoyable activity. But Picnic drives, visiting, sightseeing the neighborhood is quite different than modern CDE demands of a horse and driver.
Many breeds of light horses and ponies are easily able to make a career as Pleasure Driving animals, both in the Ring or CDE.
Draft animals and a few light breeds, like Hackney horses and ponies, are DESIGNED for being driven, managing loads they are hitched to. Shoulder angles are more upright, which helps the efficient angle of draught (pronounced draft) to move loads behind them. There are past studies done to find the “perfect” angle of draught to get the most “horsepower” available from your farm animals. Horse books of the age, told the reader what kind of build, sizes, muscling, in horse would be most useful for various jobs. You didn’t buy speed horses to do heavy plowing. You didn’t buy plow horses to make fast trips to town. Farm Chunks, crossbreds with sturdy build were popular as multi-purpose animals, light enough for trips to town in good time, enough weight for plowing in all but the heaviest ground, stood up 15-16 hands and stayed in good flesh with good grazing and a scoop of grain. Economical to own for the farmer types, if they could only afford a couple equines to work his farm.
Size of animal can make a difference to how effiecient their pulling ability is. In the past, most heavy pullers had shorter, wider, massive animals. Modern draft horses (example Budweiser Clydes), are much more leggy, taller, than what the old-time farmers would ever buy. Leggy horses would be way too big in height, expensive to feed, to be very useful on most farms.
The Budweiser Clydes would have been delivery type animals, though still overly tall for past needs, They were still extremely showy, eye-catching for advertising their owners, the beer folks. Mostly used delivering big loads in areas not too far from home base, might need some speed for outlaying areas of the city. Modern Clydes don’t look like Clydes of 100 years ago. Budweiser had greatly influenced modern breeding in the USA, because of the exposure of their hitch horses.