Continuing upon the successful SD60-series, in 1992 EMD debuted the next step in locomotive evolution with the SD70. While outwardly similar at first glance to the SD60, the D.C.-drive SD70 featured several external design refinements from the predecessor model. Battery boxes were relocated to the left-hand side walkway immediately behind the cab, a large, boxy forward traction motor blower housing replaced the angular version used on SD60s, the raised walkway duct on the left hand walkway was eliminated, and an intake for the rear traction motor blower on the left hand side of the carbody, directly under the rear radiator intake grill, was added.
Internally, the SD70 boasted improvements as well; a 16-710GB prime mover, rated at 4,000hp, was coupled to a new alternator design, the AR20. New D70TR traction motors were standard, and controlling all of this power and locomotive function was EMD’s new EM2000 microprocessor, which boasted more memory, twice the processing speed, and improved locomotive self-diagnostic capabilities compared to the processor suite used in the SD60. Even more revolutionary was the inclusion of EMD’s patented "Radial" truck design, the HTC-R. This design, which made its debut under EMD Demo SD60 #3, replaced the venerable HT-C truck, and is unique in its ability to shift, or "steer", the wheelsets laterally through curves, resulting in greatly reduced wheelset and track wear, and coupled with the new D70TR traction motors and EM2000 microprocessor, greatly improved adhesion.
Continuing with previous practice, EMD built a set of Demonstrator SD70Ms, EMD 7000-7002, all equipped with the North American safety-cab (hence the "M" in their model designation), and decked out in an attractive gray, silver, and burgundy paint scheme. These units travelled all over North America, showing off the latest technology from EMD. Eastern giant Norfolk Southern liked what it saw, and was the first to order SD70s, but with a twist; instead of having them equipped with the increasingly popular (and soon to be standard) "M" cab, they opted for standard, or "Spartan" cabs on their initial orders. The first production SD70s, NS 2501-2506, built in 1993, were quickly followed by additional units; NS 2507-2531, also built in 1993, and NS 2532-2556, built in 1994. They could be found in a variety of assignments over the NS system, and even saw run-through service on connecting roads, such as Southern Pacific, making appearances on the West Coast.
Western giant Southern Pacific also liked what it saw in the EMD Demonstrator set, and purchased 25 SD70Ms in 1994, SP 9800-9824. Intended to bolster their fleet of rapidly aging six-axle locomotives, the new EMDs were a welcome sight to SP crews and were commonly found on the "I-5 Corridor" between Southern California and Oregon, typically running in pairs on manifest trains of forest products, as well as service on hot intermodal trains.
The year 1995 saw additional SD70 customers, including Canadian National, who purchased yet another variant of the SD70-series, the SD70I, with the "I" signifying the safety cab being equipped with isolating rubber gaskets for the crew compartment, reducing the transmission of engine vibration and noise to the cab. Western road Santa Fe, always hungry for high-horsepower locomotives to speed its hot intermodal trains, purchased yet another variant, the SD75M. The SD75M featured 4,300hp, and externally was almost identical to the SD70M, save for an equipment bulge on the right side of the long hood, underneath the inertial intake grill.
As the 1990s continued, so did SD70-series orders; some repeat, while new buyers, such as Illinois Central, stepped up to the plate with orders. While mergers and takeovers would alter the ownership picture even further...such as Santa Fe’s 51 units becoming part of BNSF in 1995, and SP’s 25 units going to Union Pacific in 1996 (with the success of these units likely influencing UP’s decision to eventually order over 1,000 SD70Ms)...an even bigger change was coming. As a result of more stringent Environmental Protection Agency exhaust emissions regulations taking effect, the SD70-series needed a redesign to help meet these new guidelines. To improve engine cooling and to reduce emissions, in 2000, EMD redesigned the radiator compartment on the SD70M, flaring the radiator intake panels outward, resulting in an appearance reminiscent of EMD’s classic SD45 from the 1960s. Union Pacific, as well as Norfolk Southern, purchased SD70Ms featuring the flared radiators. The year 2002 brought about yet another change to the SD70M design; around the same time EMD introduced its FIRE (Functionality Integrated Railroad Electronics) electronic control system, the "M" cab design was revised yet again, to a more angular, easier-to-fabricate design that has, to date, remained unique to the SD70M. NS and UP both purchased units featuring this revised cab, as well as the flared radiators, giving them a dramatically different appearance in comparison to earlier production SD70Ms. As of this writing, the SD70M has been superseded by the SD70M-2 in EMD’s domestic catalog. However, the sheer number of units built of various versions...SD70, SD70M, SD70I, SD75M, etc. ...as well as their success with their respective owners, ensures that this family of locomotives will be in service for many years to come.