The United Transportation Union summarizes railroad safety laws, including the use of blue flags, on its website. The union takes its reports from the Code of Federal Regulations and summaries prepared by the Federal Railroad Administration. Besides flags, train crews can use hand lights with with blue lenses, particularly at night, to signal that rolling stock must not be moved.
The procedures affect railroad equipment on both main and side tracks. Train and yard crews are excluded when performing normal movement and operations in a yard. Yet even there, crews must display the flags when called on to service equipment beyond normal operations.
Off the mainline, a blue flag or light must be displayed at or near each manually operated switch that provides access to the track occupied by the rolling equipment being serviced. Each switch must be lined against movement to that track and locked, that is, the rails in the switch must be aligned to direct another locomotive or train away from that section of track.
For remotely controlled switches, the operator of the switch must inform the person in charge of the work crew that the switch to the track has been lined against movement and locked. The lock remains until the head of the work crew advises that it is safe.
When workers must perform service on a train or equipment already on the main line, a blue signal, either flag or light, must be displayed at each end of the rolling equipment. In the case of a train with a locomotive, a blue signal must be attached to the controlling locomotive.
If emergency repair work must be done on a train along the main, but no blue signals are available, engineers or operators must be notified and take all necessary measures to protect the crew working on the rolling equipment. Such measures include maintaining contact between the engineer of the controlling locomotive and the work crew to assure that no movements of the train or its cars occur until the work crew has completed its work and is free of danger.
Though rail yards are exempt from displaying blue flags and lights for normal operations, blue flag procedures are enforced at a locomotive servicing track or shop repair track.
