Mo. Rev. Stat. § 537.190: Fellow servant defined.

All persons who are engaged in the common service of such railroad corporation, or of any person, company or corporation operating a mine or mines, and who while so engaged, are working together at the same time and place, to a common purpose of same grade, neither of such persons being entrusted by such railroad corporation, person, company, or corporation with any superintendence or control over their fellow employees, are fellow servants with each other; except that nothing herein contained shall be so construed as to make any agent or servant of any railroad corporation in the service of such railroad corporation a fellow servant with any other agent or servant of such railroad corporation engaged in any other department or service of such corporation.