Paul, I found Mary Ann (Train) (Benjamin) Foster's death cert and it says her parents were "Celah" and "Mary" Train. I'd say that that would prove, close enough, that this particular Selah and Mary Train were her parents.
I believe that Lemuel (of course, not Selah's father Lemuel, b. Whatley, MA) who lived in Leroy, NY before migrating to Boston twp, Ionia co. MI in 1840 was Selah's son as well. He was the nephew of Sylvester (according to "Michigan pioneer experiences, 1710-1880 : with genealogical data and anecdotes unknown: The Society, 1933" and Everett's "Memoirs of the Grand River Valley" books).
I think I remember seeing someone else's tree on ancestry.com that had the other children on Selah and Mary listed (minus Mary Ann, of course).