Espee was one of the U.S. railroads ( along with Rio Grande, Burlington, Seaboard Coast Line, Western Pacific, Illinois Central, and Nickel Plate ), that made regular use of signal lights on their freight diesel locomotives. These oscillating lights were more visible to motorists and other RR personnel in adverse weather conditions; this is why most railroads also used them on passenger locomotives. However they had moving parts that required frequent repair, and ICC / FRA rules required that, if signal lights were installed, they must be operational. Espee ( and Santa Fe briefly ) tried using stationary signal lights with angled lenses that supposedly mimicked the oscillating signal lights, but actually had little effect. Ditch lights have no moving parts, just an additional circuit that allows for alternating lights when the horn is used.