I live in an area where both corn snakes and rat snakes are common. The rat snakes are nearly all black striped and have black, grey or greenish between the stripes. And occasional snake may have some orange/yellow particularly near the head. The underside starts white at the chin with increasing grey/black blotching toward the tail. The corns are varying shades of brown/orange with black around the plotches and two very faint ratsnake-like stripes and usually "dirty-wash" grey on the sides. Rat snakes often exude musk when handled. I'm not sure corns have this capability. Corns are generally more docile than ratsnakes.
The two animals in nature thus do not look alike, and have different behaviors. The big thing is even though they have ample opportunity, the two snakes rarely mate in nature. Their pheremones don't match. One has to put a male of one in a cage with a female of the other and give them no choice of who they mate. Even then, it may be necessary to trick them into mating.
Besides being obviously different in appearance, the fact that they do not mate in nature, despite having ample opportunity, is what makes them different species.
The fact that corn snakes and rat snakes are diffierent species is really no different than the fact that corn snakes and kingsnakes are different species except it is probably easier to breed a corn to a rat snake than to a king snake. All the Elaphe, Lampropeltis and Pituophis species are potentially intrahybridable. I mated a corn-king male to a corn-bull female and now I have 9 corn-king-bull snake babies.
Bigfoot