I was able to find this section of Wikipedia on the Plains Garter snake.
According to this article T.radix radix was once called "Eastern Plains Garter Snake"
Plains Garter Snake - Wikipedia, the free encyclopedia

It looks like "Easter Plains Garter Snake" is more than just a local colloquialism
It still messes with my head when I hear it.

Subspecies
Formerly two subspecies of the Plains Garter Snake were widely recognized, however, most authorities have since dropped subspecies recognition. The first, Thamnophis radix radix, was commonly referred to as the Eastern plains garter snake, while Thamnophis radix haydeni (Kennicott 1860) was considered the Western subspecies. However, the distinction between the two is weak, partly based on the number of scales and partly on slight coloration differences, with the T. r. haydeni said to have cleaner markings and more ventral and neck scales. As the two subspecies share the same habitat in many regions of the United States, further complicated by their strikingly similar appearance, many now do not recognize the two subspecies.