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Allan: I don't know if the turbo soakback pump was always there, but it was at least there by 1968. What you're seeing in the NS tables is that GP40s begin derating at 22 mph to around 2200 hp at 11-12 mph, to power-match with SD40s, via the PF17 Performance Control Module. As far as I know, all GP40s are so equipped. That's because their Minimum Continuous Speed is 22 mph, whereas an SD40's MCS is 12 mph (assuming the common 62:15 gearing). Otherwise, if you were running from an SD40 and you start going upgrade, you'd look at your ammeter as speed dropped into the teens and assume everything was cool, while back at the trailing GP40, the traction motors are melting. You wouldn't be able to have that horsepower from the GP40 anyway, except for a very brief period of time, so the PF17 module is a nice feature to have. (For those just coming in here, Minimum Continuous Speed is the speed at which a D.C.-drive locomotive can exert full horsepower without exceeding the temperature limits of the traction motors. A.C. locomotives can go to a full stall without overheating their motors.) I wi***hat 50 years ago we (Trains) hadn't obsessed over the so-called light weight of the SD7 and SD9. It turns out that very few SD7s or 9s were truly light weight -- you can usually tell them because they have a half of a normal fuel tank. Milwaukee Road and CB&Q had some, the Q going so far as to put a little "s" on the SD9 plate these units carried to signify short fuel tank. I don't know what the Q needed lightweight SDs for, but the Milwaukee Road, with what was probably the most eclectic system the world has ever seen, seemed to have a need for at least a few of everything. Who else ran string of M.U.d SW1s in everyday road service? Those must have sounded very cool. Most railroads seemed to try to ADD weight to their SD7s and 9s to make them as heavy as possible to get maximum tractive effort. I saw a lot of them with scrap freight-car wheels stacked in the back end for extra ballast. Some of them were in the nearly 400,000 lb. range, which is SD70MAC territory. Of course, you only got maximum tractive effort up to 6 mph or so, depending on whether you bought standard 62:15 gearing or the slower-speed 65:12 gearing, but if you're the DM&IR or the B&LE, that's OK. Most railroads didn't buy SD7s or 9s because they correctly couldn't figure out what on earth they needed them for. About all they were really good for was extremely heavy tonnage on a line where speed was inconsequential. Sure, they'd lug half of the yard inventory up the hill, but who would want to tie up their main line with a train going 4 mph? Thus, you see the Southern Pacific, with a raft of heavy-grade, heavy-train, one-train a day lines all over Oregon and California, going for a huge fleet of SD9s, and the Santa Fe, with practically nothing of the sort, staying away from them. You don't see the D&RGW, the other heavy-grade railroad, buying them in large quantities because their steep, heavy-train branch lines were few in number and fairly short -- or had such light rail and tight curves you didn't want a six-axle truck anywhere near them. Thus the D&RGW remains a four-axle road until the SD45 appears, and even then remains mostly a four-axle road until the coal boom of the 70s makes it need SD40T-2s. Today, the reverse is true, and four-motor locomotives aren't built anymore. You can practically sum up the last 50 years like this: until the 1960s, you haven't got enough engine to need six traction motors. After the 1980s, you've got too much engine for four traction motors. Only from about 1965 to 1980 did you really have to choose exactly which one you wanted; after that, you were better off in almost all cases with six motors. Your last question about 12-cylinder vs. 16 is interesting. I would posit that what killed the medium-horsepower locomotive was deregulation. Once that hits in 1980, loose-car traffic, branch lines, and yards have to begin going away, and with them, the need for new examples of these locomotives. You can handle what remains with downgraded main line power. Turbos didn't kill 12-cylinder engines, I don't think. Few railroads went for 12-cylinder four-axle locomotives after 1965, because you can put all of the 2000 hp a normally aspirated 16-cylinder engine develops (the GP38) into four axles with the wheel-slip technology of the time at a reasonable speed, 10 mph. If you put a 12-cylinder normally aspirated engine into the same package, you're saving very little money -- not more than 5% of the purchase price of the locomotive -- while giving up 25% of your tractive effort. So that variation was almost never made in the 1960s except a handful of GP28s. Later, the GP15 shows up. Basically, this is a SW1500 with a decent ride and a decent cab. My guess is that this model doesn't appear earlier because until that point everyone has plenty of 1200 and 1500 hp switchers with life left in them. At the point when they wear out, the GP15 becomes attractive as sort of a cross between an SW1500 and a GP38, with the flexibility of the latter and the economy of the former. I would very much like to see the purchase price and operating cost difference between a GP15 and a GP38, because the buying decision looks marginal to me. That is, unless EMD found a way to get a lot of the price out, but since there's almost nothing of the GP38 left out, I don't what that would be. A few railroads, notably Santa Fe, went for 12-cylinder turbocharged GP39s, producing 2300 hp, for reasons that remain enigmatic to me. Supposedly Santa Fe bought theirs for "high altitude branch lines," but since they had almost no such thing on the system, I couldn't see that. Besides, they seemed to run pretty much anywhere.
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