EMD SD90MAC

The EMD SD90MAC entered production in 1995 as one of the most ambitious locomotive designs ever undertaken by General Motors' Electro-Motive Division. The model was conceived around a new 16-cylinder prime mover known as the H-engine, intended to produce 6,000 horsepower and push the boundaries of single-engine diesel-electric performance. However, because the H-engine required additional development time before it could be certified for revenue service, EMD initially shipped SD90MAC locomotives fitted with the proven 16-cylinder 710G3 engine rated at 4,300 horsepower. Railroads informally called these interim units SD9043MACs, and the understanding at the time was that they could later be re-engined with the more powerful H-engine once it matured. That upgrade program was never exercised by any customer, and more than 400 of the 710-engined variants were ultimately built between 1995 and 1999. When EMD transitioned to full H-engine production in 1996, the resulting locomotives, sometimes called SD90MAC-H units, proved difficult to market. The engine's reliability record fell short of what railroads demanded, and the sheer scale of the prime mover created operational concerns: a failure in service represented a proportionally larger loss of tractive power than would occur with a smaller locomotive. Fewer than 70 H-engine equipped units were built, and deliveries were confined to just two railroads, Union Pacific and Canadian Pacific. The Canadian Pacific order had originated as a 710-engine contract and was completed with H-engines only because the transition occurred while production was ongoing. EMD also experimented with a reduced-power 12-cylinder variant designated the SD89MAC, but that effort resulted in only a single prototype with no production orders following. The legacy of the SD90MAC proved to be largely one of unrealized potential. Union Pacific, which rostered the largest fleet, began retiring its H-engine units as early as 2008, salvaging usable components while scrapping the 265H prime movers. Canadian Pacific stored its 710-engined SD90MACs for much of the 2010s before eventually committing all surviving units to a conversion program producing SD70ACU locomotives, a rebuilding effort that Union Pacific also contributed additional units to in 2021. Norfolk Southern purchased approximately 100 of Union Pacific's 710-engined examples in 2014 and similarly converted them to SD70ACUs at its Juniata Shops facility. As of the late 2010s, Union Pacific continued to operate a portion of its original 710-engined fleet, renumbered into a series previously associated with the railroad's SD40-2 locomotives.

The SD90MAC rides on a C-C wheel arrangement, with six Siemens 1TB2830 AC traction motors distributed three to each of two HTCR-2 radial self-steering trucks. The radial truck design allows the axles to steer through curves, reducing wheel and rail wear compared to conventional rigid-frame bogies. Each traction motor produces a continuous torque of approximately 12,900 Newton-meters and a starting torque of approximately 16,300 Newton-meters, with a maximum voltage rating of 2,183 volts. The 710G3-equipped variant generates 4,300 horsepower at 900 rpm, while the 265H-equipped variant produces 6,000 horsepower at 1,000 rpm. Starting tractive effort is rated at 185,000 pounds-force for the 710-engined version and 200,000 pounds-force for the H-engine version, with a locomotive weight of approximately 420,000 pounds in either configuration. Maximum operating speed is 70 miles per hour. The SD90MAC's cab is mounted on shock absorbers to isolate it from the mechanical vibrations of the main frame, a feature intended to improve crew comfort during long road assignments. The locomotive also features a wide radiator section spanning nearly the full width of the carbody, a design characteristic shared with the SD80MAC and later models such as the SD70ACe. Later production SD90MAC-H units incorporated a revised Phase II cab with a redesigned nose offering improved forward visibility. The H-engine itself, designated the EMD GM16V265, represented an entirely new engine architecture rather than an evolution of the long-established 710 family, which contributed significantly to the reliability difficulties that ultimately limited the model's commercial success and led EMD to discontinue production in 2005 in advance of tightening EPA Tier 2 emissions requirements.

Models by: Athearn