by John Hathaway

The larger this number the more recent the model. The F-units found In Southern California include the F-3, the F-7, and the F-9. The letter "A," or no letter, following the first two digits of the model number indicate that engine has a cab for the engine crew. The letter "B" indicates the engine is a booster unit only with no cab and controls.

The EMD "S" units are similar in external design to the F-units, but are much longer, ride on six axles, and are powered by two diesel engines.

In the field of road switchers, EMD's "GP" series refers to four axled models. A long line of "Geeps" are on the rails today, from the earliest GP-7's to the high horsepowered GP-40s. Union Pacific and Santa Fe both have the booster variety of "Geep," and these models also have the letter "B" behind the classification number.

The EMD "SD" aeries of road switchers are the six axled models. In most cases the GPs and SDs are basically the same engine with the exception of number of axles.

The "DD" series, of which Union Pacific and Southern Pacific both operate, ride on a total of eight axles and are powered by two diesel engines within a single carbody.

With the GE locomotives, the classification number can virtually identify the engine sight unseen. All GE numbers begin with the letter "U," a number and then the letter "B," "C," or "D." The "B" refers to four axled engines, the "C" is a six axled model, and the "D" is an eight axled job. The middle number is the horsepower rating of the locomotive. For instance, a GE U-25-B, is a four axled, 2500 hp locomotive. The U-25-C is basicly the same engine, except it rides on six axles.

The ALCo classification system is not as simple as GE's and EMD's so we'll deal with individual models as we get to them. A fine source of information is Jerry Pinkepank'a Diesel Spotter's Guide available for $3.50 from any model railroad hobby shop.

Next month we'll have a photo section of the different specific models of diesels that can be found on the mainlines of the Southwest Corner.

The art of diesel spotting as opposed to steam spotting of the olden days is a much more sophisticated one for the railfan. Contrary to what the die-hard steam enthusiasts say, all diesels are not the same, but often it does take a bit of fore-knowledge to distinguish even the radical differences between two different models of locomotives. But the differences are there, and the hobby of train watching can be enhanced with a bit of knowledge about these differences.

When identifying a certain locomotive there are a number of basic steps to follow. For the time being we will limit ourselves to the larger road engines of the three major railroads here in the Southwest Corner (Santa Fe, Union Pacific, and Southern Pacific).

The first step is to determine the manufacturer. There are only three builders represented within the area we are covering today, and each has its own distinquishing characteristics in external design.

Far and away the leader in sheer numbers is the Electro-Motives Division of General Motors (EMD). Of the three major builders EMD accounts for some 60% of the domestic market. EMD offers the railroads a whole line of standardized models and for the most part does not deviate into specialized exotic designs.

Trailing a distant second is General Electric, a relative newcomer into the mainline road engine field. All three lines have GE representatives on their rosters.

No longer in the locomotive producing business, but still well represented, is the American Locomotive Company (ALCo). ALCo was the last, and most successful, of the steam locomotive builders that transcended into the diesel field.

There are two methods by which a particular model of locomotive might be classified by a railroad. The first is, of course, the manufacturer's own catalogue number. This catalogue number, actually a combination of letters and numbers, offers the railfan a number of important clues in identifying different diesel locomotives,

With EMD all manufacturer's numbers beginning with the letter "F" refer to four axled cab, or "streamlined," units. Following the "F" is a single number.