Cataloged from 1972–1986, the SD40-2 was the last hurrah of EMD’s reign as the locomotive builder for America’s railroads — a dynasty that began in the 1930s and vanquished Alco, Baldwin, Lima and Fairbanks-Morse. For more than a decade, the six-axle, 3000 hp SD40-2 was virtually the standard new-purchase engine for North American freight service, until General Electric dethroned EMD as the top-selling diesel builder in 1983.
Tasked with creating an upgrade to the already-solid SD40, EMD’s designers created a locomotive that looked a lot like its predecessor but was better in almost every way. The major visual change from the SD40 was a three-foot longer frame with distinctive front and rear “porches,” required to accommodate the new high-adhesion HT-C trucks — which improved both tracking and pulling power. Under the hood, upgraded traction motors and alternator, along with tougher pistons, piston rings and bearings in the 16-cylinder turbocharged model 645 motor, made the SD40-2 10-15% more reliable than the SD40 it replaced.
The most significant change, however, was a major advance in diesel technology: solid state electronics. Gone were the hardwired circuitry and the maze of switches, contacts, interlocks and relays that had characterized all earlier diesels and led to a well-known saying in locomotive shops: “With a steam engine, it took five minutes to find a problem and five hours to fix it; with the new diesels, it takes five hours to find the problem and five minutes to fix it.” The transistors, printed circuit boards and other solid-state components that formed the electrical guts of the new -2 engines were both easier to maintain and more reliable, and paved the way for the computerized controls that would later introduce the third generation of diesel technology.
The SD40-2 turned out to be the apex of second-generation diesels, and the perfection of EMD’s 645 diesel motor. Viewed as an industry benchmark for reliability, the SD40-2 became EMD’s all-time best seller, with 3,949 regular SD40-2’s delivered. Including variations — like Southern Pacific’s famed “tunnel motors” and units with elongated noses that housed remote-control electronics — total production exceeded 5,700 locomotives, sold to 24 American and 6 Canadian roads, plus railroads in Brazil and Mexico. Even today, more than three decades after the last SD40-2 was delivered, more than 1,000 units are still rostered by the Big Six Class 1 railroads in North America.