Here is an excellent article from Bloodhorse regarding stamina in Triple Crown race winners.
Sources of stamina
https://www.bloodhorse.com/horse-racing/articles/85211/sources-of-stamina
Stamina Lines In American Thoroughbreds
http://www.sport-horse-breeder.com/stamina-lines.html
This should be resurrected each and every year when the naysayers come out to disrespect Triple Crown contenders.
The above stands in contrast to articles like this...
American Pedigrees Need Fresh Infusion of Stamina
https://www.thoroughbredracing.com/articles/american-pedigrees-need-fresh-infusion-stamina
Though I would suggest all pedigrees need a fresh infusion of stamina and speed to keep the breed durable.
Or articles like this that suggest American racehorses are no longer fast...
California Chrome's Attempt to turn back the clock in the Kentucky Derby
https://www.thoroughbredracing.com/articles/kentucky-derby-california-chrome%E2%80%99s-attempt-turn-back-time
And this article offers interesting background into American thoroughbred horses spring pedigrees
Understanding Sprinting Through Thoroughbred Lines In American Sport Horses
http://www.sport-horse-breeder.com/american-thoroughbred.html