Massachusetts General Laws Part I. Administration of the Government (Ch. 1-182) Ch. 149, § 18H

The employer shall provide blue stop signs for use by day and blue lights if night work is necessary, and shall cause them to be displayed before employees are permitted to work in, upon or under any standing railroad car or cars under conditions where unanticipated movement of such car or cars might endanger employees or equipment. The blue signals shall bear the word STOP in white letters not less than four inches in height on a blue background, and shall be placed on the track in a clear and unobstructed place. The signs shall be not less than twelve inches by fifteen inches in size. The signals shall be placed at a height of three to five feet, and not less than ten feet from either or both ends of the cars, as necessary to provide protection. The blue stop signals shall be placed and removed by one of the employees working in and about the cars. This employee shall be selected, instructed and directed by the employer in the safe procedures involved in placing and removing the blue stop signals.