A blue signal, displayed at one or both ends of an engine, car or train, indicates that workman are under or about it; when thus protected, it must not be coupled to or moved. Other equipment must not be placed on the same track so as to intercept the view of the blue signals, without first notifying the workmen. Each class of workmen will display the blue signals and the same workmen are alone authorized to remove them.
When emergency repair work is to be done under the engine, or under or about cars in a train, and a blue signal is not available, the engineer must be notified by a member of the crew and protection given those engaged in making the repairs. Train or engine must not be moved, nor air brakes applied or released, until all employees are out from under or from between cars, and the engineer so advised by the same employee.