Final answer:
The most preferred method for transporting large numbers of horses is by using trailers, as they offer flexibility, safety, and direct routes compared to other methods such as rail or air transport.
Step-by-step explanation:
The preferred method for transporting large numbers of horses today, especially over long distances, is by trailers. More specifically, these are special horse trailers attached to trucks designed for safe and efficient equine transport. Trailers allow for flexibility in scheduling, route planning, and are more readily equipped to provide immediate care or rest stops for animals. In contrast, rail transport was historically significant for longer distances but is less frequently used due to the need for specialized infrastructure and less direct routes. Air transport is used for international travel or for particularly valuable horses, but it is not as common due to its cost. Horse-drawn carriages are impractical and outdated for moving large numbers of horses and were replaced as cities industrialized and internal combustion vehicles became prevalent. Thus, the use of trailers is now the more preferred method for horse transportation.