EMD SD24

The EMD SD24 was introduced in July 1958 as the product of Electro-Motive Division's effort to bring turbocharged power to its six-axle road switcher line. Built at EMD's La Grange, Illinois facility, the SD24 holds the distinction of being the first EMD production locomotive to incorporate a turbocharged diesel engine, entering production roughly sixteen months before the four-axle GP20 followed suit. EMD initially anticipated strong interest from the Duluth, Missabe and Iron Range Railway, going so far as to paint the first demonstrator unit in that road's colors, but the DMIR ultimately declined to purchase the type and the demonstrator was instead delivered to the Union Pacific Railroad. A total of 224 units were constructed between 1958 and March 1963, of which 179 were conventional cab-equipped locomotives and 45 were cabless B units built exclusively for Union Pacific. Sales of the SD24 proved only moderately successful by EMD's standards, though the locomotive served on several prominent railroads. The Atchison, Topeka and Santa Fe Railway was among the largest customers, eventually accumulating a fleet of eighty units. As the locomotives aged through the 1970s, reliability concerns emerged, particularly involving electrical systems, and the added maintenance demands of the turbocharged powerplant became difficult to justify in secondary service. This prompted a wave of rebuilding programs by various owners. The Santa Fe undertook the most extensive effort, rebuilding its entire SD24 fleet between January 1973 and January 1978 into what the railroad designated the SD26, upgrading the prime movers with EMD 645 power assemblies and replacing the electrical systems wholesale. The Illinois Central Gulf Railroad pursued a different approach, rebuilding a mixture of SD24s alongside other C-C locomotives into 2,000-horsepower SD20s between 1979 and 1982, removing the turbocharger and upgrading the electronics to Dash 2 standards. Union Pacific experimented with three individual units to evaluate potential fleet-wide upgrades, though none of these experimental modifications were ultimately replicated across the broader roster. Despite its modest commercial performance, the SD24 occupies an important place in diesel locomotive history. Its introduction of EMD's mechanically assisted turbo-compressor established the foundation for all subsequent turbocharged EMD products and set the developmental course that led to the high-horsepower six-axle locomotives that came to dominate North American railroading in later decades. One SD24 was acquired by Kennecott Utah Copper for industrial service, representing the lone example ordered outside of general railroad use.

The SD24 rode on a C-C wheel arrangement, meaning six powered axles across two three-axle trucks, and drew its 2,400 horsepower from an EMD 567D3 prime mover equipped with the company's newly developed mechanically assisted turbo-compressor. This turbocharger differed from conventional designs in that a gear train and overrunning clutch allowed the engine itself to drive the compressor during startup and at lower throttle positions where exhaust energy was insufficient to spin the turbine at adequate speed. As power demands increased, the overrunning clutch disengaged and the unit functioned as a true exhaust-driven turbocharger. The arrangement delivered full rated power regardless of altitude, a meaningful operational advantage over the Roots blower-equipped SD18, which used the same engine displacement but produced only 1,800 horsepower and lost performance at elevation. With 400 horsepower per axle, the SD24 was constrained by the traction motors available at the time rather than by the engine itself. The standard fuel tank carried 1,200 US gallons, but virtually all production SD24s were delivered with the optional 3,000-US-gallon tank. To accommodate this larger tank beneath the frame, the air reservoirs were relocated to the roof behind the cab, where their long cylindrical shape earned them the informal nickname torpedo tubes among railroaders. Buyers could specify either a high or low short hood, with the short hood oriented to the front as the default configuration. The Burlington, Southern Railway, and the first EMD demonstrator were among the few delivered with high short hoods, while the remainder left the factory with low short hoods featuring a characteristic downward slope toward the nose. Optional equipment available at the time of order included dynamic brakes, steam generators, winterization packages, multiple unit controls, and varied fuel tank capacities, though the overwhelming majority of units were delivered in broadly similar configurations.

Models by: Atlas