The term "Three Rail Scale" was coined as a way to express a seemingly logical endpoint in the evolution of three rail O gauge trains toward greater scale realism. Three rail o gaugers now have available models made to 1/48 scale proportions. Along with that trend, has come greater attention to detail -- both in general, e.g., add on grab irons, and road specific variations, e.g., placement of vents, screens, number of windows and so on. The logical conclusion to this trend is to make equipment that is in all respects a scale model of the prototype, much as one would find on a two rail O scale layout, but capable of operating on three rail track. In addition to the things just mentioned, that means eliminating some of the unrealistic attributes of traditional three rail operation, such as oversized couplers and floating pilots and abbreviated handrails on diesels. (Whether to continue with traditional three rail wheels is a subject for separate treatment; the idea remains to use three rail track, which does not immediately lend itself to a more scale wheel).
In 2001, MTH Electric Trains released the Premier Line Dash 9. This engine is a good example of the trend toward scale realism in three rail O gauge. It replaced the earlier Premier "Dash 9," which actually was just a Dash 8 body shell fitted with different truck side frames. The two prototypes might look alike to the casual observer, but are indeed different models. The then newly tooled Dash 9 was a distinct step in the direction of meeting the needs of the O gauge customer interested in greater scale realism. The new model was a correct scale 73 feet long and generally was an accurate representation of the Dash 9. MTH also included a number of detail variations, including placement of lights and number boards depending on the roadname offered. At the same time, the model failed to reflect a number of detail variations, that increasingly are reflected in scale-oriented models released by MTH and Atlas O.
Pictured below are a pair of Dash 9s, updated for "Three Rail Scale" operation. The pilots are fixed and more scale like Kadee couplers installed. In addition, the original hand rails have been removed and replaced with new full length ones.