The Alligator People (1959)

A newlywed couple sit in a train. The husband receives a frantic telegram. He gets off at a station to make a phone call, the train pulls away without him on it, and that's the last his wife sees of him. Years later after a long search she finally tracks him down on his family's southern estate where she discovers that a failed medical treatment has turned him into an alligator mutant.
https://archive.org/details/AlligatorPeople1959