It’s actually a lined snake. Tropidoclonion Lineatum. Lined Snakes are superficially similar to both the Common Gartersnake (Thamnophis sirtalis) and the Plains Gartersnake (Thamnophis radix) in appearance, but can easily be differentiated by the presence of the half-moon markings on the underside.
Snake was found as a neonate. Photographed as an adult in captivity several years later.