A guy I know in Australia is doing fantastic work with his Etrakka heart rate/GPS training device. So fantastic in fact, that one of his charges recently won a Grade One event.
Better yet, he has provided the actual heart rate vs speed chart from the week prior to this huge victory - results that prompted the trainer to enter the horse when he had originally been thinking of races further in the future.
But, when they are peaking you run them, every horseman knows that - the difference is this time the peak was scientifically discovered: making it a concrete fact, not an opinion.
Much more info at: