I'd love to get to the point where I can comment on a thread like this, but I just learned how to tell the difference between a GP-40 and an SD-40. An SD39 is just asking too much.
It's not that hard, Chip, at least with EMD locomotives. You've figured out the main difference - GP means B-B or four powered axles and SD means C-C, or six powered axles.
After that, look at the exhaust(s). One large rectangular stack closer to the cab from the middle of the long hood and it's a turbocharged engine, two (or sometimes four) smaller oval or round stacks roughly centered on the hood and it's a normally aspirated engine.
Number, size and position of fans is a big indicator of what you're looking at, too. The fans you see over the middle of the long hood on some engines are dynamic brake cooling fans, but dynamic braking is an option, so some locomotives don't have them at all. This option has nothing to do with determining the model of locomotive, though, but it's important to distinguish radiator fans from the dynamic fans when making an ID on a locomotive.
The radiator fans are at the end opposite the cab and usually have large rectangular grilles under them on the hood sides. Some variations on EMD locomotives did not have the typical round fan and grille arrangement and instead featured an angled grille pair on top and large see-through intake grilles on the sides just above the deck. This variation is common to the SD45T-2 and SD40T-2 of the six axle variety and to the GP15, GP15T and GP15-1 in the four axle variety (I'm not counting the switcher type for this example).
Other successful models were improved throughout their production runs and these improvements often led to new phase designations (which I won't get into here to avoid complicating things more than they are at this point), but could also lead to new model designations. A good example is the set of changes adopted to the EMD catalog in 1972 known as the "Dash 2" line. The 1972 locomotives were different enough from their earlier counterparts that a new model designation was required, so "SD40" became "SD40
-2" and "GP38" became "GP38
-2" and so on. This set of improvements was largely internal, but the most obvious is the "water sight glass" which is an oval window on the engineer's side at the front of the radiator, which allowed an inspection of the radiator water level without opening a hood door. There are other differences, but some are too subtle to pick out in the average photograph and others, such as truck type, varied with the individual railroad's specifications of equipment or the use of recycled parts.
Now, to get down to the difference between an SD39 and any other SD type locomotive of similar vintage, let's look at the fans. An SD39 has two radiator fans and the much more common SD40 has three. Like the SD40, the SD39 is turbocharged so it has one large exhaust. The SD38 also has two radiator fans, but it is normally aspirated, so it has at minimum two smaller exhaust stacks. Why only two on the SD39 and three on the SD40? The 40 line used the 16 cylinder diesel and the 38 and 39 line used a 12 cylinder diesel, so with four fewer cylinders to generate heat, only two fans are needed.
Change the number of powered axles to four and we're talking about GP38s, GP39s and GP40s. The spotting features are the same, as far as exhaust type and number of fans are concerned, so once you have this down it's easy to spot the difference and ID a locomotive right away.
There are numerous locomotive models produced by EMD over the years and virtually any of them can be identified based on the criteria I listed above. Again, these are:
- Number of powered axles
- Number and type of radiator fans
- Number and type of exhast stacks
- Dasj 2 spotting features, such as water sight glass
I know you're into the steam era, but if you want to know how to ID any EMD locomotive quickly and easily, I can give you the skinny on the various models.
