Good ol’ Charley Eckhardt, a regular on the Ask the Marshall Forum, stepped up and answered this one for me:
“Normally a stage was pulled by what was known as a six-up hitch. The wheel team, those at the rear of the team, were the largest, strongest of the six. The wheelers started the coach moving. The swing team, the middle team, were smaller than the wheelers, but likewise well trained. The lead team, also smaller than the wheelers, were the least well-trained of the hitch. They could look through a collar, and they reined fairly well, but otherwise they could be a mite flighty. The better-trained and larger wheelers and the better-trained swing horses held them in check.
“Some lines used mules; in Texas, those were called the ‘jackass mail.’ Most of ’em were trained to work two-up, not six-up, and there are horror stories about jackass mail coaches getting run away with by the hitches.”