A blue signal signifies to railroad operators that workers are on, under, or between railroad rolling equipment. A blue signal display means a clearly distinguishable blue flag or blue light by day and a blue light at night—if it’s not clearly distinguishable, it doesn’t meet requirements. Signals must be placed at each derail, and must be visible to employees seated at the controls of a locomotive. When the signal is displayed, equipment may not be coupled to or moved, rolling equipment may not pass a displayed blue signal, and other equipment may not reduce or block the view of a blue signal.
According to the U.S. Department of Transportation’s “Guidelines for the Specifications of Blue Safety Flags in Railroad Operations” report, a supplier must provide a light that is uniform in appearance and that has a blue appearance that falls within the range of a safety code. Suppliers can use blue LEDs to meet these requirements—the use of a plastic blue filter over a white light is not sufficient.