From the dawn of dieselization to the early 1990s, every diesel electric was powered by DC traction motors. Today, however, virtually all new road diesels have AC traction motors, which are more reliable, simpler to build and maintain, and can start a heavier train with the same horsepower. That revolution began in earnest with the SD70MAC.
In 1991-92, after years of research and development, EMD fielded America's first production AC road engines, a quartet of 3800 horsepower SD60MACs. Burlington Northern stepped up as a test site for the engines and fell in love with them. A year later, it placed a sight-unseen order for 350 new 4000 hp SD70MACs - despite the additional half-million dollars or more that AC electronics added to each engine's price - and the AC revolution was on. Slightly late to the party, General Electric introduced its first AC engine, the AC4400CW (shown elsewhere in this catalog) the same year, but production would not ramp up until a year later.
From 1993 through 2004, when new environmental regulations produced the upgraded SD70ACe, EMD sold 1,109 SD70MACs. The largest fleets were and still are owned by BN (now BNSF), which ultimately bought over 800, and CSX, with over 200 purchased. In addition to AC traction, a big selling point of the SD70MAC, and the entire SD70 series, was its self-steering trucks, a feature GE could not match at the time. EMD's HTCR ("high traction, three-axle, radial") truck could steer into curves, greatly reducing wear on both track and wheels; it was widely recognized as superior to the trucks on competitive GE diesels.
About Norfolk Southern No. 1800
When an automobile is recycled, nothing much remains but molten metal. When a modern diesel locomotive is recycled, however, the frame and much of the exterior is re-used, while internal organs are replaced or upgraded. In an era when a new freight locomotive can cost upwards of $2 million, recycling old power makes both economic and ecological sense.
Begun in 2015, Norfolk Southern's "DC to AC" program upgrades 20-year-old diesels with more horsepower, lower emissions, more comfortable cabs, and conversion to AC electric motors. Outshopped in September 2018, engine no. 1800 is the lead engine in a group of 25 1990's-era SD70s rebuilt as SD70ACCs by Progress Rail in Muncie Indiana. Its yellow color symbolizes Caterpillar, the parent company of Progress Rail; most other engines in its group wear NS's black and white Thoroughbred paint scheme.