The Mars catalog and how it listed its products is still not entirely known although some designations which are include the following: "SB" referred to Sealed Beam, "WR" was short for a White-Red, and if a single-digit number followed this was said to refer to the number of bulbs used. There was also normally a trailing number that typically described the specific model type. What makes its system even more confusing, however, is its hazy references. The SB-R-250 highlighted above used by the Nickel Plate, for instance, used a white lens even though the "R" described it as having a red lens. The confusion, however, goes even further beyond this. The company also had a wide range of products that would not fit the above designation such as the OS-250-RE-14, AWR-12-2, WR-5000-A, and others. In any event, it seems railroads had a particular model or type that they preferred on their locomotives.
During the diesel era a Mars Light looked much more attractive as it was flush-mounted on the nose of cab units, particularly Electro-Motive's E and F series. On the Southern Pacific the company was famous for its use of Mars' vertical SBW/R-2-300/1 models that would sit directly atop the short, high-hood of its early road-switchers such as GP7s, GP9s, SD7s, and SD9s. Over the years the Mars Light came in a wide range of orbiting displays from the common figure-eight pattern to vertical or horizontal oscillating motions all of which were designed to clearly and easily catch the attention of bystanders or motorists to keep them clear of railroad rights-of-way. During this time the lights featured a single bulb or up to three in different colors (normally red or white).
While the Mars Light was the most common form used during its era the company did have competition from the Pyle-National Company, which used its Gyralight that functioned somewhat similarly except that instead of oscillating it moved in an elliptical pattern. Between the late 1960s and early 1970s Mars opened a location in Naples, Florida and while there the company was purchased by the Trippe Light Company, which renamed itself as Tri Lite Mars or Tri Lite, Inc. Today, the operation is again back in Chicago and still produces warning lights and sirens although no longer designed for railroad applications. Because Mars Lights often required added maintenance most railroads stopped using them by the 1980s. However, they can still be found on many historic locomotives at museums and excursion railroads, more as a novelty than anything else.
For more reading about the history of diesel locomotives consider Mike Schafer’s Vintage Diesel Locomotives which looks at virtually all of the classic builders and models from Alco PAs to early EMD Geeps. If you’re interested in classic FMs, or diesels in general, this book gives an excellent general history of both. You may also want to consider the book Evolution of the American Diesel Locomotive by author J. Parker Lamb. As the title implies the book looks at the history and development of the diesel locomotives, covering 200 pages, from its earliest beginnings to the newest designs and models operated today.