Re: 8th Inf. Reg't Missouri (Union)
I believe this service card actually refers to the 4th West Virginia Volunteer Infantry. The enlistment and muster dates match for this unit raised in Mason City Virginia (West Virginia). The unit was furloughed and disbanded at the time of Cain's service in Missouri and many of the former 4th VA men ended up re-enlisting in the 2nd WV Infantry.
Post to the West Virginia or Ohio boards may give you more info. larry Stevens has a bibliography of the unit posted on the web I believe.
As to how he came to be in Missouri, I've no clue.