If you are entirely new to identifying diesel locomotives, pick up a good book (one of the best to get started is Brian Solomon's "American Diesel Locomotive") on the subject to get yourself familiar with the various locomotive manufacturers with the most common names being: the American Locomotive Company (ALCo), Baldwin-Lima-Hamilton Locomotive Works(BLH), Fairbanks-Morse (F-M), General Motors' Electro-Motive Division (EMD), and General Electric (GE). This type of book will usually provide descriptions, illustrations of locomotives and photographs.


Today, only the latter two companies still produce diesel locomotives, although at least a few models built by all of the companies listed above still operate in some capacity. In any event, for purposes of time this article will only highlight the general differing spotting features among different manufacturers and model types. Diesel locomotives have three basic types; switchers, road switchers, and cab units which usually feature some type of streamlining and a full-length hood with no "porch" or walkway alongside the locomotive.;
, Switchers are usually small locomotives built with one purpose in mind. That purpose is movement of cars in a specific area. However, this didn't limit them to just that. Switchers are also used to run short freight in order to unload and load cargo from nearby industries. Quite a handful still operate today, especially in industrial settings.

Among switchers, models built by EMD, as are most of their locomotives in general, feature smooth, distinct lines and contours with the cab set to one end of the locomotive with a signature conical stack(s) protruding from the top of the hood. Except for a few models, EMD's switchers were short, typically only around 45 or so feet in length. Models included: SW-1, NW-2, SW-7, SW-9, SW-1200, and SW-1500.
GE's switchers were distinct and very short. While their larger 70-ton model featured an end-cab design, their switchers were commonly center-aligned. They were highly sought for industrial work since their very small dimensions allowed them to negotiate tight curves and clearances often found within plants and other industrial workplaces. Models included the 44-ton and 45-ton.
ALCo's switchers, as with most of their models, are defined aesthetically by rounded corners and roof lines. Their "S" and "T" models featured end-cabs and one single stack protruding from the roof with the latter model featuring a notched nose for the number boards. While technically a road-switcher, Alco's very popular RS series was often used by railroads in switching service. Featuring a cab offset to one end and a long hood with its trademark rounded edges and lines, the RS series is still universally recognized today.
BLH's S-12 featured an end-cab design with a very long sweeping front hood and extended front step leading out of the cab. Never very popular, only a handful of these machines still operate today.
F-M's switcher models included the H-10-44 and the H-12-44, which featured an end-cab design that rose flush with the top of the long hood, with the edges rounded off.
 Road-switchers are basically what the name suggests. They usually work freight trains, moving goods between yards. However, their general design allow them to be safely operated as switchers in those yards. In regards to the first and second-generation models, EMD and GE locomotives are the most easily recognizable.

EMD's designs, as usually found on first- and second-gen units, are typically very clean in look with an angled and pointed short front hoods with a finished angled-off appearance to the rear of the long hood. Perhaps the most distinguishable feature is the angled protruding dynamic brake housing centered on top of the long hood and a smooth, semi-streamlined fuel tank. Such models include the GP-7, GP-9, GP-20, GP-30, GP-40, and GP-50. More modern units have a much more drastic look compared to older units. One key difference between them is a stubby, short front hood that could be said to be streamlined. They look comparatively like GE's newer models.
GE's older models were about the general shape as EMD's GP-7 and GP-9: boxy. The Universal series exemplifies this description. However, like modern EMD designs, newer GE units are vastly different from their predecessors, as they are very boxy in appearance with clean, crisp lines, sharp angles, stubby, short front hoods and often-"winged" radiator housings in appearance protruding from the rear of the long hood (one of a modern freight diesel's most distinguishing features).
ALCo locomotives are typically easily identified by their rounded edges and notched features on the front and rear of the locomotive. Common models included: RS-2, RS-3, RSD-4/5, RS-11, and RS-15. Their large Century series road-switchers are very bulky and tall in appearance, but can usually be identified from GEs and EMDs by their rounded cab roofs, short, stubby short front hood and protruding front number plates directly above the windshield. These models included: C420, C424, C415, and C628.
F-M's most popular road-switcher was the H-24-66, the Train Master. It was very tall, bulky and boxy featuring an offset cab that rose flush with the roof line. Similar, but somewhat smaller locomotives included the H-16-44 and the H-16-66 models.
 In North American railroading, cab units are generally streamlined locomotives that have one cab at the extreme end, offering a great view of the road ahead and an extremely limited view behind. Cab units are generally considered as road-only units due to the extreme lack of rearward visibility, and as so, are rarely seen switching a yard if they are even used as so.

EMD's first- and second-gen cab units were easily the most popular as their E and F series diesels sold by the thousands. These models are easily recognizable by their clean, "bull dog"-like noses and portholes along their flanks. Both models featured very streamlined car bodies making them all ideal for passenger service although the E series was particularly equipped for the purpose. Their latest cab unit diesel appeared on the market not too long ago as the F125, presumably numbered after its max speed. The shape is reminiscent of GE's Genesis series, with a beveled edge at the cab end of the locomotive.
Alco's cab units may not have been as popular but were striking nonetheless. Their FA (for freight service) and PA (for passenger service) models featured very automobile-like styling with a long, sweeping front nose and rounded windshields. The PA was generally considered the most striking and has often been credited as the most beautiful diesel locomotive ever built.
FM's “Consolidation Line” was a short line of diesels which introduced the company to the cab unit world. The model, built to both freight and passenger specifications featured a short, almost stubby front nose that was somewhat rounded but also pointed. It only sold a few hundred units and was never very popular. Another F-M cab unit was the Erie-built, named as it was built in Erie, PA.
Get a book (optional).
Understand what switchers are.
Get to know road-switchers.
Consider cab units.
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