Always note the locomotive cab, this is usually a distinguishing feature among locomotives.
To properly identify various diesel locomotive (officially known as diesel-electric locomotives) models is a bit of an acquired skill that can take a fair amount of time, especially if you are completely "green" and know little about them. Before we begin, if you are entirely new to identifying diesel locomotives I would first suggest picking up a good book 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 Locomotive Works, Fairbanks-Morse (FM), General Motors' Electro-Motive Division (EMD), and General Electric (GE).
Today, only the latter two companies still produce diesels for the railroading sector 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 (usually found switching cars in yards), road switchers (typically used in everyday freight service they provide more power than switchers), and cab units which usually feature some type of streamlining and a full-length hood with no "porch" or walkway alongside the locomotive.
Among switchers EMD's models, as our 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.
General Electric'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 small frame allowed them to negotiate tight curves and clearances often found within plants and other industrial workplaces.
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.
As for Baldwin-Lima and FM switchers the former featured an end-cab design with a very long sweeping front hood with an extended front step leading out of the cab. Never very popular few of these brutes still operate today. FM's lone model was the H-12-44 and featured and end-cab that rose flush with the top of the long hood, which was rounded off along the edges.
Among road-switchers, in regards to the classic first and second-generation models EMD and GE's are the most easily recognizable. EMD's designs, as usual, are typically very clean in look with a angled and pointed short front hoods with a finished angled-off appearance to the rear of the long hood. Perhaps their most distinguishable features is the angled and protruding dynamic brake housing centered on the top of the long hood and a fuel tank that is smooth and streamlined.
GE's models are vastly different and very boxy in appearance with clean, crisp lines, a short, stubby short front hood and a radiator housing that is often "winged" in appearance protruding from the rear of the long hood (one of GE's most distinguishing features).
Alco's models are typically easily identifiable by their rounded edges and notched features on the front and rear of the locomotive. 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.
FM's only popular road-switcher was the H24-66, the Train Master. It was very tall, bulky and boxy featuring an offset cab that rose flush with the roof line.
In terms of cab units EMD's were easily the most popular with their E (for passenger service) and F (for freight service) series selling 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 carbodies making them all ideal for passenger service although the E series was particularly equipped for the purpose.
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 the most striking and as often been credited as the most beautiful diesel locomotive ever built. FM's “Consolidation Line” was its only dab into 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.
Today's newest diesel models come from only GE and EMD, the latter now its own company known as Electro-Motive Diesel. The newest EMD unit is the SD70ACe with its most recognizable feature being the extended front nose and offset "winged" radiator housing to the rear. GE's newest model is the ES or Evolution Series and features now-common GE trademarks like the angled, but clean sweeping front nose and radiator housing to the rear of the long hood. It should be noted that all of today's newest locomotives now feature the "safety cab" which gives models a more streamlined look but no longer allowing the crew to step out onto the front of the locomotive walkway directly from the cab (they must exit from a door on the front of the nose).
For more information about diesel locomotives and the various models and types please click here.