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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.
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