Massachusetts General Laws Part I. Administration of the Government (Ch. 1-182) Ch. 160, § 155

A railroad corporation, in moving traffic between points in the commonwealth, shall not use any locomotive not equipped with a power driving wheel brake and appliances for operating the train brake system; nor run any train in such traffic unless a sufficient number of cars in it are so equipped with power or train brakes that its speed can be controlled by the engineer of the locomotive which is drawing such train, without the use of the common hand brakes by the brakeman. Said brakes and brake appliances shall be tested or examined periodically by the department, and shall be so tested or examined while the cars are in motion and under both normal and emergency conditions. A record of each such test or examination shall be kept by the department. When such corporation has equipped a sufficient number of its cars with such power or train brakes, it may lawfully refuse to receive from connecting lines of railroad any cars used in such traffic which are not sufficiently equipped with such power or train brakes as will work and readily interchange with the brakes in use on its own cars.