TheEMD SD50 is a 3,500-horsepower (2,610 kW)diesel-electric locomotive built byGeneral Motors Electro-Motive Division. It was introduced in May 1981 as part of EMD's "50 Series"; production ceased in January 1986. The SD50 was a transitional model between EMD'sDash 2 series which was produced throughout the 1970s and themicroprocessor-equippedSD60 andSD70 locomotives. A total of 431 were built.^[1]

EMD SD50
An SD50 operated byKansas City Southern Railway
Type and origin Power type Diesel-electric Builder Electro-Motive Diesel General Motors Diesel Clyde Engineering Model SD50 Build date December 1980 – July 1987 Total produced 431
Specifications Configuration: ​  • UIC Co'Co' Gauge 1,435 mm (4 ft 8+1⁄2 in) Length 71 ft 2 in (21.69 m) Fuel type Diesel Fuel capacity 3,500 U.S. gal (13,000 L) Lubricant cap. 283 U.S. gal (1,070 L) Coolant cap. 250 U.S. gal (950 L) Sandbox cap. 56 cu ft (1,600 L) Prime mover EMD16-645F3B RPM: ​  • Maximum RPM 950 Engine type V16diesel Aspiration Turbocharged Alternator main : AR16/D18 (3,500), AR11 (3,600) Traction motors EMD 87Y (3,500), EMD 87A (3,600) Cylinders 16 Transmission Diesel-electric Gear ratio 70:17 MU working Yes Loco brake Westinghouse,Dynamic brake Train brakes Air braked
Performance figures Maximum speed 70 mph (110 km/h) Power output 3,500 hp (2,610 kW) at 950 rpm; increased to 3,600 hp (2,685 kW) November 1984.
Career Operators SeeOriginal buyers Disposition Some in service, some retired, one preserved

The SD50 was produced in response to increasingly tough competition fromGE Transportation, whoseDash 7 line was proving quite successful with railroads. While EMD'sSD40-2 was a reliable and trusted product, GE's line included locomotives up to 3,600 hp (2,685 kW) with more modern technology, as well as very competitive finance and maintenance deals. EMD responded throughout the SD50 program by offering discounts on large orders.

The GM-EMD locomotives that immediately preceded the SD50, the 3,600 hp (2,685 kW)SD45 andSD45-2, used huge,20-cylinder engines that consumed large amounts of fuel and suffered from reliability problems when first introduced. Demand for the 45 series dropped sharply after the 1970s fuel crisis. The SD50 used an updated version of theV16645 from the SD40-2, uprated to 3,500 hp (2,600 kW) at 950rpm from 3,000 hp (2,240 kW) at 900 rpm, and uprated again in November 1984 to 3,600 hp (2,685 kW). This proved to be a step too far; the 50 series models were plagued by engine and electrical system problems which harmed EMD's sales and reputation.

Compared with their predecessors, the SD50 had a longer frame and a substantially longerlong hood. In addition, the resistors for thedynamic brake grid were moved from their location on previous models above theprime mover to a new, cooler location in front of the engine compartment air intakes, closer to the electrical switchgear. This increased their separation from other systems, simplifying maintenance for the prime mover and the electrical system.

One former SD50 engineer praised the locomotives'wheel-slip control system and dynamic braking power, stating "they were able to give more adhesion than aSD40-2".^[2]

The SD50 is powered byV16 16-645F3 series diesel engine driving either an EMD AR11A-D14 or an EMD AR16A-D18 tractionalternator. The power generated by the traction alternator drove 6 EMD D87traction motors rated at 1170 amps each.

The SD50 was available with multiple traction motor gearing ratios and wheel sizes, the most common of which was the 70:17 ratio with 40-inch (1,016 mm) wheels, which allowed for a top speed of 70 mph (110 km/h). Other gearing options for the SD50 with 40-inch (1,016 mm) wheels included 69:18 for 76 mph (122 km/h), 67:19 for 82 mph (132 km/h) and 66:20 for 88 mph (142 km/h). The SD50 was also available with 42-inch (1,067 mm) wheels.

The SD50 was also available with either HT-C trucks (identified with a hydraulic shock on the side of the middle axle of each truck) or the earlierFlexicoil trucks. Some investigators implicated the HT-C truck in derailments ofAmtrak'sSDP40Fs, soConrail chose the Flexicoil C trucks for their SD40-2s and their first order of SD50s—the only customer to do so. The controversy surrounding the HT-C truck was eventually disproved, and Conrail chose HT-C trucks for their second order of SD50s and SD60s.

TheSD50S ("short frame") were prototype units built in December 1980. They were shorter than production locomotives by approximately 2 feet (0.61 m). There were six SD50Ss built, all of which were sold to theNorfolk & Western and eventually passed toNorfolk Southern. They were withdrawn in the early 2000s as non-standard. Two were rebuilt in 2008 as "SD40E" models by Norfolk Southern'sJuniata Shops, along with several standard length SD50s.

The SD50S designation was also used for five locomotives built by EMD Australian licenseeClyde Engineering,Adelaide forHamersley Iron.^[3] Shorter than production SD50s, they were equipped with a special double cab roof for insulation against the hot Australian desert sun in thePilbara region. Withdrawn in November 1995, they were sold toNational Railway Equipment Company and exported to the United States in February 1999 and used in national lease service.^[4] They were subsequently sold to theUtah Railway in June 2001.^[5][6] In June 2017, 6063 and 6064 were sold to theChicago, Fort Wayne & Eastern Railroad, 6064 was relocated byGenesee & Wyoming to theIndiana & Ohio Railway and renumbered 5016 and 5017.^[7]

TheSD50F was a Canadiancowl unit version equipped with a "Draper taper" (inset section aft of cab for limited rear visibility). Sixty were built for theCanadian National Railway as road numbers 5400–5459. Early engine problems resulted in a temporary down-rating to 3,300 horsepower (2,500 kW). Eventually the units were re-rated to 3,500 horsepower (2,600 kW); however, like their U.S. cousins, they continued to suffer from relatively low reliability, frequently suffering engine, power assembly, and crankshaft failures.

A number of SD50s have been rebuilt into the equivalent ofSD40-2s. TheDash 2 features are already contained within the SD50. The units are derated from 3,500 hp (2,600 kW) at 950 rpm to 3,000 hp (2,200 kW) at 900 rpm. This is actually a quite simple change to the locomotive'sWoodward PGE engine governor.

• In late October 2022, theIllinois Railway Museum acquired formerCNW 7009, believed by the museum to be the first preserved SD50.^[2]

• List of GM-EMD locomotives
• List of GMD Locomotives
• Chicago and North Western 7009

  Media related toEMD SD50 locomotives at Wikimedia Commons

• Lytle, Stan.EMD SD50 list. Retrieved 16 February 2005.
• TrainWeb.com.The Unofficial EMD homepage. Retrieved 7 January 2005. Contains fairly complete builders' records for EMD production.
• Union Pacific Railroad Locomotive Department (1994).Locomotive Diagram Book.Union Pacific.
• Electo-Motive Division of General Motors (1980).SD50 Operator's Manual 1st Edition November 1980.Electro-Motive Diesel.