Union Pacific was the largest purchaser of the SD70M model, with over 1400 eventually on the roster. With such a large group of locomotives, there are many variations between orders.
The 5127-5231 number group-some of the last that were produced for the UP-are depicted here. These late production units feature several key characteristics including 4-panel flared radiators (which have never before been offered before), phase II cabs, & late profile fuel tanks.
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.