"Temporalis" (the coastal plains milk) is currently (previously, now?) considered to be an intergrade between a triangulum and an elapsoides. This doesn't mean that if you find a temporalis in NC, one parent was a triangulum and one was an elapsoides. It means that the parents were a mixture. Along the periphery of an intergrade zone, the snakes more strongly resemble the "pure" subspecies (not really a valid idea in triangulum, but used for simplicity) due to a stronger genetic influence from them. Eastern milks and scarlet kings aren't the best example of how things work, so I'll break this down into parts and use western milks for an example. Now about subspecies...
Subspecies are not distinct entities in the way that species are, and many scientists STRONGLY suggest that their use be discontinued because of the false implications the subspecific name invokes. Species (reproductively discrete units, by one definition - there are others) tend to show clinal variation in characteristics over their geographic range. When these characteristics remain relatively stable over a defined area, a subspecies is named. Where the characteristics of one of these named populations blends with the characteristics of another named population, an intergradation zone is said to occur. The populations freely interbreed (intergrade) and exchange genes.
The above is more easily exemplified using L. t. gentilis and L. t. multistrata than triangulum and elapsoides. The characteristics of the milks living in southern WY and northern Colorado are a blend of gentilis and multistrata traits. Some animals will more strongly resemble one or the other, and some will look like a perfect mix - that's the fun of genetics! As you move North or South, the influence of multistrata or gentilis, respectively, becomes stronger.
Triangulum and elapsoides are quite different animals... I think we can all agree. Some recent genetic work I've heard about (not yet published) indicates that they do NOT intergrade and that "temporalis" don't share genetic material from both. We could expect a revision of Lampropeltis triangulum in the next few years that splits elapsoides and gives it species status. It's not my work and is yet to be published, so that's about all I can say on the matter.
Sorry if this was long, boring, and confusing. Here's a few photos to make up for it.
Wyoming multistrata

Colorado gentilis

Intergrade gentilisXmultistrata

-Cole