The two other subspecies are not Mexican, but live even more southwards...

Thamnophis marcianus bovalli - from southwest Nicaragua up to northern Costa Rica.
Thamnophis marcianus marcianus - from southwest Kansas to the west up to southeastern California and southwards, viavia the north and east of Mexico, up to northern-Veracruz. In Tehuantepec (Oaxaca, Mexico) lives an isolated population.
Thamnophis marcianus praeocularis - from the Yucatan-peninsula, via Belize and northeastern Guatemala up to western Honduras.

I do not know the physical differences.