Mo. Rev. Stat. § 389.920

Every corporation, lessee, owner, operator or receiver or other person owning or operating a railroad in the state of Missouri shall provide and maintain in a plainly marked accessible location on all passenger train cars, cabooses, each locomotive unit capable of independent operation and all motorized on-track work equipment used in railroad operations, which weigh in excess of four hundred pounds, a first aid kit conforming to such requirements as the division may, by rule, prescribe.