Mo. Rev. Stat. § 389.840: Cars to be equipped with air or power brakes.

From and after the first day of January, 1908, it shall be unlawful for any person, persons, company or corporation, operating any line of railroad, in whole or in part, in this state, either as owner, lessee or receiver, to operate any train of cars over such line of railroad between stations within this state, unless at least seventy-five percent of the cars composing such train shall be equipped with air or power brakes, and any of the remaining twenty-five percent of such cars composing such train shall be so equipped with such air or power brakes; they shall be so associated and connected that brakes thereon can be used, managed and operated by the engineer of the locomotive drawing such train.