Mo. Rev. Stat. § 389.810
From and after the first day of January, 1908, it shall be unlawful for any person, persons, partnership or corporation, operating any line of railroad, in whole or in part, within this state, either as owner, lessee or receiver, for the purpose of moving freight or passengers between points wholly within this state, to use upon such line of railroad any locomotive or engine to move such train of cars over such railroad without having such locomotive or engine equipped with power drive wheel brakes and fully and properly equipped with air brake appliances so that the engineer operating such locomotive or engine shall have the means of fully and completely controlling the air brakes on the cars attached to said locomotive and engine without recourse to hand brakes, except in cases of emergency.
§ 389.810, RSMo