View source for General Motors Electro-Motive Division SD series

The '''General Motors SD (Special Duty) series''' of locomotives are six-axled locomotives were built by [[Electro Motive Division|EMD]] in America and [[General Motors Diesel Division|GMDD]] in Canada. While it was originally marketed as a light-footprint unit for running and switching on lines with low axle loading, railroads soon took advantage of the additional tractive effort they offered due to their ability to be more heavily loaded and the additional traction motors. By the 1970ies SD-series units were outselling GP-series units by a fair margin. They were offered alongside the [[General Motors Electro-Motive Division GP series|GP-series]] of 4-axle units and the [[General Motors Electro-Motive Division SW series|SC/SW-series]] of switcher locomotives, but are now the only freight units catalogued by EMDD for use in North America.

==SD Families==

*'''SD7 - 1952-1953'''

The SD7 was essentially built and designed as a 6-axle GP7, right down to the 1500hp 16-cylinder 567B engine. Other than the additional axles and length required by the larger trucks, the major spotting difference was the use of a single-section radiator, with 4 36" fans grouped together at the end of the long hood. This would remain a standard SD spotting feature until the introduction of the SD80MAC in 1994.

*'''SD9 - 1954-1959'''

Realizing that they might just be onto something, EMD quickly offered the SD9 as a replacement to the SD7. There were virtually no external differences between the two, the only major difference being the 1750hp 16V-567C used in the SD9.

*'''SD24 - 1958-1963'''

In 1958, EMD finally catalogued a pairing in the SD-series. The SD24 was introduced as a premium SD model, as a pairing of sorts with the GP20. Internally, the main difference between the GP20 and SD24 was that the additional traction motors of the SD allowed for an increase in output from the main generator, and thus the same 16V-567D2 was bumped up to 2400hp.

The SD18 was introduced in 1960, replacing the SD9. Again it was mechanically identical to the GP18, right down to the 1800hp 16V-567D1.

*'''SD35 - 1964-1966'''

Much like the GP35, the SD35 heralded the evolutionary end-of-the-line of the venerable EMD 567 design. And much like the GP35, it would also herald the change to the modern, angular carbody design used until the introduction of the SD80MAC in 1994.

Two members made up the SD35 family: the 2500hp 16-cylinder 567D3A-powered SD35 and the 1800hp 16V-567D-equipped SD28. Interestingly, only 6 SD28's were sold in North America, half of the total production run of 12.

*'''SD40 - 1966-1972'''

The SD40 was the first EMD 6-axle family to use the then newly-developed 645 platform. Much as it's GP40-family bretherin, it featured a host of members, the theory being that there was one model suitable for virtually every railroad on the continent. Other than the base model (16V-645E3, 3000hp), 3 other models (with numerous variations of each) were available: the SD38 (16V-645E, 2000hp), SD39 (12V-645E3, 2300hp) and the SD45 (20V-645E3, 3600hp).

Much like the GP-series, towards the end of production, EMD offered an AC-transmission option on all of the models, with the notation "DC" being added to the model name for generator-equipped units and "AC" added to alternator-equipped units, although only the SD38AC was purchased in any quantity.

*'''SD40-2 - 1972-1986'''

The SD40-2 family was nothing if visually disappointing when it was announced. Other than the newly designed HTC trucks and the longer frame required to contain them, they looked virtually identical to their predecessor units outside. They did contain a host of internal differences that would help increase EMD's market share through to the early 1980ies, including:
-New truck design that improved adheasion
-Modularized and solid-state control circuitry that was easily replaced if a part failed, and was more robust than previous designs
-Bolted battery box covers that wouldn't come loose
-Longer rear traction motor duct that was more effective at cooling the rear traction motors
-Long-range dynamic braking
-Improved design central air intake filters and radiator screens

Until passed by the GE Dash9-series in 2005, the SD40-2 family was the most popular selling series of locomotive in North America, and the SD40-2 itself still remains the most-sold unit on the continent, with over 5000 versions built.

The only member of the SD40 family not to make it to the -2 series in 1972 was the SD39, although Milwaukee Road did order a small number of unique, light-footed SDL39's in 1974.

*'''SD50 - 1981-1986'''

While EMD was trying to get their next-generation engine sorted out, they decided to make one last push from the 645-series powerplant that had served them so well since 1966. A new turbo-supercharger was developed to wring a bit more horsepower from the block, a new radar-equipped traction control system was installed and a new carbody layout, with the dynamic brake grids located immediately behind the cab, and thus the SD50 family was born.

Any praise was short-lived however. It seemed that the new "F3" turbo-supercharger pushed the 645 block beyond what it was designed to handle, and many railroads ended up de-rating their units, installing older "E3" turbo-superchargers or mothballing almost-new units in exchange for more orders of SD40-2's or new GE power.

The SD50 family had two members: the base 3500hp (later 3600hp) 16-cylinder 645F3-equipped SD50, and a 12-cylinder 645F3-equipped, 2800hp SD49 (none of which were built). 

*'''SD60 - 1985-1995'''

Realizing that the SD50 was not going to win back market share, the EMD rushed the development of its replacement powerplant, and in 1985 announced the SD60. Virtually identical in appearance and performance, it's major advantage was use of the 3800hp 16-cylinder 710G3, its greatly improved reliability and its reduced fuel consumption. It also brought EMD kicking and screaming into the computer age: each unit was equipped with a host of microprocessors which did everything from monitor wheel slip and engine speed to making sure that the headlights weren't burnt out. While it brought back some sales, many roads were still wary of EMD power and decided to buy GE, or in a couple of cases, purchase and rebuild older EMD power.

Once more a 12-cylinder variation was available (SD59, 3000hp) although once again, none were built.

There was also 4 SD60MAC prototypes built that demonstrated on Burlington Northern, but they were proof-of-concept vehicles, and not meant for mass production. They did introduce a number of concepts used on later models, such as AC traction and modern radial (steerable) trucks.

*'''[[General_Motors_Electro-Motive_Division_SD70_series|SD70]] - 1994-2005'''

The 70-series family was the first not to have have a companion 4-axle locomotive. Railroads for many years had been progressing towards heavier and longer trains, and had found that 4-axle locomotives simply didn't have enough grip on the rail to be able to drag them up the hills (although were more than happy on flat terrain).

Powered by a 4000hp 16V-710G3B (later upgraded to the G3C turbo-supercharger), the SD70 and AC-traction SD70MAC were the modern-day GP9 - a loco for all seasons and conditions. According to EMD's sales literature, 3 SD70's could replace 5 SD40-2's in intermodal service, and 2 SD70MAC's could replace 4 SD40-2's in bulk service.

The truth is that the SD70 didn't do too badly in sales until UP's enormous 1000 (later upped to almost 1400) SD70M order in 1999. All in all, just over 3000 70-series units were built, with UP accounting for over half.

*'''SD80 - 1995-1996'''

The SD80MAC was a bit of a mutt. Designed as a stop-gap to allow EMD time to refine their upcoming 265H engine, it featured many of the external spotting features of the later SD90MAC and yet featured the electronics and mechanical systems of the SD70MAC. Powered by a massive 20-cylinder 710G3B, it also suffered the unfortunate fate of being birthed a little too late: many of the railroads who ordered them either cancelled them outright (in the case of C&NW) or switched their orders to SD90MAC's when EMD announced that they were going to bring it to production in 1995 (in the case of CPR). Even Conrail's order, in progress at the time of the split, was changed to SD70M's for NS and SD70MAC's for CSX.

Visually, it is very different than any other EMD product before. The isolated cab was offered as a standard feature (as it was with all AC-traction EMD locos), but it featured a new, lengthened carbody with huge flared radiators, and with the dynamic brake moved to the very end of the carbody. 

Just 30 were built: the first 28 of Conrail's 125-unit order, and 2 demonstrators that were sold to Conrail soon after.

*'''SD90 - 1995-1999'''

Up to mid-1990ies both GE and EMD were locked in a war of epic proportions: the horsepower war. Each side attempted to out-do the other in succession until in 1995, both announced monsterous 6000hp behemoths: the AC6000CW and the SD90MAC.

In EMD sales literature, the SD90MAC was the way of the future: its new powerplant was capable of meeting all current and projected future emissions standards, it claimed an enhanced computer control system capable of doubling the tractive effort of an SD40-2 in all conditions, and it boasted of an enhanced reliablity and duty cycle that allowed for servicing every 6 months.

In practice, it came somewhat short of it's claims. The computer control system worked well when it was dry but was finicky and had to be restarted frequently, and the enhanced duty cycle went out the window when it was found that the engine wasn't tested thoroughly enough, and blown turbochargers and stress cracks were being found in almost-new engine blocks at an alarming rate.

And this was despite EMD (and GE) taking the unprecidented step of offering "upgradeable" units: locomotives that were complete SD90MAC's save for the engine, which in EMD's case was the reliable 16V-710G3C, and designed to have the 6000hp 16V-265H engine dropped in when it was perfected. In practice, not one of the hundreds of upgradeable units built were ever upgraded - in fact, one 6000hp GE unit was converted to an older powerplant in an effort to improve its reliability.

*'''SD70M-2/ACe - 2004-Current'''

When it became apparent to EMD that the 265H was not going to be perfected in time, efforts were made to develop the changes necessary to the 710-series to make them compliant with the new emissions regulations to take effect January 1, 2006 (Tier II).

Visually, they are similar to the SD90MAC - they share the same "Phase II" angular cab design, and very similar carbody and radiator details. The engine is similar to the one used in the SD75M - a 16V-710G3C-T2 putting out 4300hp - with some additional software and sensors to monitor emissions in real-time. The major difference is in the length - the SD80/90 frame is a shade over 80 feet long, whereas the SD70M-2/ACe is 6 feet shorter.

Internally, it is a bit of a mish-mash - featuring the same (but updated) control systems as the SD90MAC, it uses a 70-series engine and main alternator, coupled to newly designed traction motors. The ACe has the largest difference of all, in that Siemens is no longer the invertor/traction motor supplier for EMD: it is now Mitsubishi.

Until EMD is finally able to work out the bugs with the 265H design, it would appear that the 710-series is here to stay.