The headlight burning on dim, has ALWAYS been considered a substitute, for markers, on the rear of a train. Long before DP was ever used, Light engines, and helpers have had them since the rules about continuously burning a headlight came into existance. Locos usually had markers installed on them, and they were used, where required, ALONG with a dim burning headlight. I have seen "blue" signals, along the ROW (ever had to call a signal "high blue"?) I know of several locations, on several railroads, where the green signal somehow shows a blue light. We still regard it as green, but it is clearly blue. I am sure any engineer, with enough miles under his belt, will recall one somewhere. I don't know if it's a lens installed from the wrong source, a lens/lamp combination, or what. Markers are accepted in the "red to orange" range. I have seen yellow, as well, but not as widely distributed as orange, or red. Blue provides protection, for Mechanical forces, whether carmen, or roundhouse persons, and recently added, were trackworkers, in sidings. The old green, white and red, head end markers, were for designating extras, second sections, and last following section, of the first section. They were not for marking the end of the train, although Conrail did utilize high visibility red marker lights, on most of the road diesels, right up to the end. These were used in conjuction with dim headlights. They are not, as some have speculated, used for marking the rear of a train, i.e. marker lights. (those Conrail engines excepted) Flags and lights, of the same colors (red, white and green) have always been carried on locos, until recent times. Markers, either on the hack, or hung off the last car, have always been red, or orange. (with the occasional yellow) Some ancient roads, used to also have green lenses, on the markers, to be turned facing the rear, when a train had cleared the main, for a following train, or to indicate a following section was the last section, of a train, to go by. Regards