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There were three versions of the SD90. The SD9043MAC, a 4,300 hp "convertible" locomotive designed to eventually take the 265H engine. 309 of these versions were sold to UP and 60 to CP. NS has now (September 2014) bought 100 from UP via EMD. None were ever converted. Then there was the initial 6,000 hp SD90 sold to UP only; there were 40+ of these unit outshopped. Then finally in 2000 the SD90MAC-H II was produced. 40 of these were sold to UP and 4 were sold to CP. The CP order was originally to be 20 units but problems plagued the 265H prime mover.
It was also the main competitor to the GE AC6000CW but it unfortunately shared the same fate.
Union Pacific and Canadian Pacific were the only two North American Class 1 railroads to purchase the SD90MAC-H. No SD90MAC-H's are in service in the US. UP retired their SD90MAC-H Phase 1's. The SD90 series locomotives production could no longer continue into 2005 because of EPA Tier 2 regulations.
The SD90 did not fare well in the US. However, some former UP SD90MAC-HII's have been exported to FMG in Australia as an equivalent to SD9043MAC's.
After the success of the SD70MAC in 1995, EMD began experimenting with higher-horsepower, AC-traction diesel locomotives to begin a new era for the North American locomotive market. After the SD70MAC's initial success, EMD became interested in developing an even greater advantage of high-tech motive power. Hence, the development of the SD90MAC and its lower-horsepower counter-part; the externally-similar SD80MAC, delivered to only Conrail, CNW had 15 SD80MAC's on the books, but the UP-CNW merger cancelled the order. The SD90MAC was successful on paper (such as having "steerable" trucks, advanced computer microprocessors, and an advanced cooling system) yet EMD's competitor; GE, developed a slightly more successful, popular, and more light-weight AC-traction diesel locomotive; the AC6000CW, shortly a year after which dominated in popularity and salesof 6,000 hp Locomotives.
They were first ordered and demonstrated on the Union Pacific (UP), and practically served as their "mighty come-back" for having powerful locomotives operate on their system once again for heavy-duty revenue freight service (like with their "Big Boy" and "Challenger" types of articulated steam locomotives in the past; besides their other numerous types of powerful double-diesels and gas electric turbines). Though, not long after the type or model's introduction or early on in production, the Canadian Pacific Railway grew fond of the 6,000 hp concept. The company went ahead with purchasing such type to haul trains throughout the steep grades on their various routes throughout Vancouver, British Columbia in Canada.
Although originally intended for having 6,000hp, the actual engine (the EMD 265-H or simply "H" engine) wasn't fully developed in-time during the overall introduction and beginning of production. Thus, early units were fitted with a standard EMD 710 series engine generating 4,300hp and served as a temporary "filler" until the "H" engine became available (and were then eventually offered); making them "convertible": being capable of easy conversion or modification (though, the Union Pacific; being the first customer, dealt with such issue).
Almost from the start the EMD SD90MAC's 6,000hp 16-cylinder 265 "H" engine (known as the EMD 265-H) began to reveal some of its numerous flaws. Such as "teething" issues with the cylinders, crankshaft failures, as well as having the entire engine shut down due to overheating the entire cooling system (although the cooling system was originally guaranteed to be successful for the "H" engine; but wasn't.
Hence, the latter converted 4,300hp SD90MAC units being referred to as the "SD9043MAC", aside from non-265-engined units who never received the 6,000 UP has begun the process of renumber all 8000 series SD9043MAC's to make room for new ES44AC's.
The SD9043MACs will reside in the 3400 and 3500 series.
As of September 2014 NS is to buy 100 former UP SD9043MAC's via EMD. The numbers will be 7229-7329. The units will be pressed into service but later be rebuilt to essentially SD70ACe's in SD90 car bodies. They will then be reclassified as SD70ACu.
As of September 2013, an EMD/ELMX SD90MAC-H II has been placed under Cummins ownership. The unit will receive a QSK95 prime mover (this is first QSK95 powered locomotive) that will be installed by Sygnet Rail Technologies. The locomotive will be EPA Tier 4 compliant, and rated at 4,200 hp. The unit will test on INRD starting in 2014.
Later on in production, EMD began producing several experimental versions of the SD90MAC (one leading towards the development of the SD70ACe); such as the SD89MAC. Yet none were successful, but inspired ideas and the design for the SD70ACe and having an improved cab (which was eventually used as the actual cab for the SD70ACe; aside from its DC counter-part). Thus, creating the "Phase 2" SD90MAC-H which had an improved cab, a permanently-installed "H" engine (being non-convertible) and having different rear hood design which helped with previous cooling issues for the cooling system.
Union Pacific SD9043MACs 8000-8308 are currently being renumbered to 3470-3775.
Union Pacific SD9043MAC 3483 was repainted in the new Z stripe paint that's used on other Union Pacific locomotives, especially the repaints.
The EMD SD90MAC is considered to be one of the more unsuccessful locomotives produced by EMD.