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The EMD SD90MAC is a 6,000 hp (4,470 kW) C-C diesel-electric hood unit locomotive produced by General Motors Electro-Motive Division. It is, with the SD80MAC, one of the largest single-engined locomotives produced by that company, surpassed only by the dual-engined DDA40X and the Pennsylavania Railroad T-1 class duplex-drive 4-4-4-4 steam locomotives. Currently, a few SD90MACs are being rebuilt into SD70ACu's by Norfolk Southern, while a few others are still in service.
UP 8540, an SD90MAC Phase II
The SD90MACs feature radial steering trucks with AC traction motors and an isolated safety cab which is mounted on shock absorbers to lessen vibrations in the cab. The SD90MAC, like the SD80MAC, SD70ACe, and SD70M-2, has a wide radiator section, nearly the entire width of the locomotive, which along with their size makes them easy to spot.
The SD90MAC was introduced in 1995, along with the far more successful SD80MAC locomotives. The SD90MAC was designed to utilize the new 16-cylinder H-engine, while the SD80MAC was designed to use the 20-cylinder version of the existing 710G engine. However, technical problems with the 6,000 horsepower (4,500 kW) engine resulted in the first locomotives being shipped with 4,300 hp (3,210 kW) 16-cylinder 710G engines, making them similar to the SD70MACs. These locomotives were given the informal model designation SD9043MAC by railroads that purchased them with the option to re-engine them with 6,000 hp (4,470 kW) engines when they became available. This upgrade program, however, was never taken advantage of by SD90MAC buyers due to reliability issues with the newer engine. Over 400 SD90MAC locomotives fitted with the 4,300 hp (3,210 kW) 710 engine were built.
In 1996, Electro-Motive Diesel entered full production on their 6,000 hp (4,470 kW), 16-cylinder H-engine, and all SD90MACs made from then on used that for its prime mover. Locomotives fitted with this engine are sometimes referred to as SD90MAC-H locomotives. Later versions of the SD90MAC-H feature a Phase II cab, with a new nose which offers higher visibility from the cab than the old nose. The SD90MAC-H did not prove popular with railroads and less than 70 were built, including Electro-Motive Diesel demonstrator units. Since the SD90MAC-H had such a large prime mover, it didn't offer the same operational flexibility as smaller units, limiting its possible customer base to only the largest railroads. Also, since the H-engine was a new design it hadn't reached the same level of reliability as EMD's previous engine. The low reliability on such a large engine was an especially bad combination since the loss of one engine in a train meant the loss of a larger percentage of pulling power than had a smaller engine failed. In the end the SD90MAC-H was only delivered to two railroads, the Union Pacific Railroad and the Canadian Pacific Railway. The Canadian Pacific locomotives were part of an earlier order for 710-engine equipped SD90MAC locomotives that was still in production when Electro-Motive Diesel switched over to the H-engine.
Prime mover (EMD 710):
Prime mover (EMD 265):
- EMD V16 EMD GM16V265
- Power Output - 6,000 hp or 4,470 kW at 1000 rpm
- Idle - 200 rpm
- Full speed - 1000 rpm
- 6 Siemens 1TB2830 AC motors mounted 3 each on 2 HTCR-2 radial self-steering trucks.
- Rated output - 855 hp or 638 kW
- Gearing - 83:16
- Wheel size - 45 in (1,143 mm)
- Max revolutions - 3,435 rpm
- Starting torque - 16,300 N⋅m or 12,022 lbf⋅ft
- Continuous torque - 12,900 N⋅m or 9,515 lbf⋅ft
- Maximum voltage - 2,183 V
Performance (parenthesis indicate H-Engine equipped specifications):
- 9100-9160 (SD9043MAC): Units are now up for sale in "as is" condition. Some units have been placed into freight service to help combat a motive power shortage.
- 9300-9303 (SD90MAC-H): These were scrapped in Lachine, QC. 
- 100-139 (SD9043MAC) (Lease Fleet) 10 units traded to NS for 15 MP15DC's.
- 8500, 8523-8561 (EMD Lease fleet), Former Union Pacific 8500, 8523-8561, currently stored. All units leased to Kansas City Southern in 2008 on one-year lease. Several to the Buffalo and Pittsburgh Railroad upon returning from KCS lease. All units currently stored. Nine were sold to Fortescue Metals Group in Western Australia. Units have since been sold to many different owners, primarily for scrap.
- 9001-9013, 9025 (SD9043MAC) (Long-term lease from CEFX).
- 7229-7338 (SD9043MAC); NS purchased 100 units in September 2014 from EMD/Progress Rail. Those units are ex-UP. 10 units were acquired from CEFX in June 2016, in exchange for 15 MP15DC's. All units will be converted to SD70ACus.
- 8000-8308 (SD9043MAC); to be renumbered 3470-3775 to make room for new GE ES44AC's. Most, except 8, of these have been stored. In September 2014, Norfolk Southern purchased 100 of these.
- 8500-8561 (SD90MAC-H. Fleet Retired, some after less than 5 years of service. 8500, 8523-8561 to EMLX Lease fleet, 8501-8522 renumbered 8911-8931. Retired in late 2008-2009. Sold to Metro East Industries in Alorton, Illinois. Scrapped mid to late 2009)
- 901-904 (SD90MAC-H)
- 901 - 265H-engine was replaced with a 710 engine Tier 1 on March/13 at FMG Thomas Yard - Port Hedland / Western Australia
- 902 - 265H-engine was replaced with a 710 engine Tier 1 on Nov/14 at FMG Thomas Yard
- 903 - 265H-engine was replaced with a 710 engine Tier 1 on October/14 at FMG Thomas Yard
- 905-909 (SD9043MAC): Rebuilt from SD90MAC-H for use in Australia at the Juniata Shops
- PRLX8527 is now FMG 905
- PRLX8530 is now FMG 906
- PRLX8531 is now FMG 907
- PRLX8541 is now FMG 908
- PRLX8554 is now FMG 909
- 910-917 (SD90MAC-PII): Rebuilt for use in Australia at Mayfield, KY shop
- All locomotives in this series are currently stabled at Thomas Yard - Port Hedland.
- All 8 locos unloaded at Thomas Yard in December/14
- PRLX8553 is now FMG 910
- PRLX8548 is now FMG 911
- PRLX8550 is now FMG 912
- PRLX8557 is now FMG 913
- PRLX8525 is now FMG 914
- PRLX8538 is now FMG 915
- PRLX8543 is now FMG 916
- PRLX8547 is now FMG 917
As of January 2005, the SD90MAC is no longer in production due to the Environmental Protection Agency's Tier 2 locomotive emission regulations, although EMD may be able to get the H-engine approved at some later date. However, the People's Republic of China recently announced an order for 300 JT56ACe locomotives, which also use the H-Engine as its prime mover. A portion of the locomotives will be assembled in kit form by China's own Dalian Locomotive Works. These units were reportedly capable of meeting the EPA's strict regulations and began delivery in the latter half of 2007.
Due to reliability issues FMG units 910 to 917 are stored and deemed not be used. Purchased to be used as "slave" units they were more offline than they were in use.
FMG looking to fit 910-917 to same spec as 901-909. 916 currently in shops
On August 1, 2008 Union Pacific Railroad announced that it would be retiring 21 of the SD90MAC locomotives. According to sources  common parts will be used to maintain the railroad's 710-engined SD90MAC fleet. The 265H prime mover will be cut up or exported. The only part that is non-reusable will be the frame. These units, in the 8900 series will be removed from storage in Denver, and be sent to East St. Louis, Ill., in small batches to be dismantled.
CPR SD90MAC-H locomotives 9300-9303 (listed as SD90MAC6000s) were up for tendered sale in 2009. The bidding ended March 31, 2010. While they were originally thought to be shipped overseas, they were eventually moved to Quebec for scrapping.
CPR SD90MAC 58 locomotives remaining in the 9100-9160 series (9129, 9133 & 9138 scrapped Nov. 2012) had been put up for sale as of November 2012 and again in Jan. 2013 with no offers being made. These units are still stored, mainly in Winnipeg, Manitoba as of the Winter of 2018.
In September 2014, Norfolk Southern purchased 100 4300 hp SD9043MACs from Union Pacific with the intent to rebuild them into SD70ACUs at the Juniata Shops. The last 4 SD90-MACS were taken delivery to Norfolk Southern on October 3, 2015 running parallel to Western Ave in Chicago IL.
In 2015 Cummins rebuilt a single SD90AC with its QSK95 engine, designed to meet Tier 4 emissions standards. Sygnet Rail Technologies and Brookville Equipment Corporation performed the conversion. The rebuild was designated HTL4200AC. The locomotive, numbered CECX No. 1919, had been Union Pacific No. 8559.
- The Locomotive Place. Retrieved on January 13, 2005.
- The Unofficial EMD homepage. Retrieved on January 13, 2005.
- Rails, American. "American Rails EMD SD90MAC". American Rails. American Rails. Retrieved 13 August 2017.
- Johnston, Howard (2005). Jane's Train Recognition Guide. London: Collins. pp. 440–441. ISBN 0-06-081895-6.
- Shop, The Diesel. "EMD SD90MAC Data Sheet". The Diasel Shop. The Diesel Shop. Retrieved 13 August 2017.
- Clark, Peter (2012). An Australian Locomotive Guide. Rosenberg Publishing. p. 220. ISBN 9781921719554.
- "Cummins Tier 4 freight locomotive headed to Indiana for testing". Trains. November 10, 2015. Retrieved November 11, 2015. (subscription required)