the cowboy had a few horses match the speed of the train car that was carrying the nuclear waste, and used the horses to basically slingshot the train car in a different direction from the town after it had made its way to the gap in the tracks where the cowbay earlier detonated a small charge
the maneuver worked, but somehow they accidentally contaminated more of the surrounding area than intended, and a whole bunch of wild pokemon died. a catastrophically large amount.