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Locomotive Phases - EMD SD70MAC

The SD70MAC was introduced in 1993 following two years of tests with four SD60MAC prototypes. It was EMD's first production locomotive with AC traction motors and, along with the rest of the SD70 series, was the first to use EMD's radial-steering, bolsterless "HTCR" truck design. Burlington Northern (and later BNSF) accounted for the majority of production, ordering nearly 800 units over the following seven years. The precise control and ruggedness of AC traction motors allowed for superior performance in both low-speed lugging and dynamic braking, and AC traction has since become ubiquitous on modern 6-axle freight units.

The SD70MAC was produced alongside the DC-powered SD70M (and related models, such as the SD70 and SD75I). Although the two models shared many similarities, there were some differences aside from the traction system. The SD70MAC rode on a slightly longer underframe (74' 0" vs. 72' 4" over the coupler pulling faces) with extra hood length visible behind the radiator intakes. While DC models used two separate traction motor blowers, the SD70MAC continued to use the central blower design of earlier EMD models, with a duct on the left side extending from the hood and running along the walkway. The inverters for the AC traction motors were housed at either end of the hood, one under the dynamic brake intakes and one under the rear half of the radiator intakes.

Most SD70MAC's were rated at 4,000 horsepower, with later units uprated to 4,300. Most were also built with EMD's isolated cab, which can easily be identified by the seam running around the nose and behind the back wall of the cab. While the SD70M received revised model designations for both changes (SD70I for the isolated cab, and SD75M / SD75I for higher-horsepower versions) the SD70MAC apparently did not. No SD70MAC's were built with the older standard (non-safety) cab used on the SD70.

SD70MAC production occurred largely prior to the end of 2000, and the last SD70MAC's ordered by the Alaska Railroad and CSXT after 2003 featured canted radiator intakes for a revised split-cooling system. In 2005 the SD70MAC was replaced by the SD70ACe, which met Tier 2 emissions standards and which was built on a substantially revised platform.


These phases are of my own making. Other than the canted radiator intakes denoting Phase 2 units, most external changes were relatively minor. Some of the changes that appeared on Phase 2 units in 2003 had been introduced gradually in concurrent SD70M production over the previous 2-3 years.

Phase 1a 1b 1c1 1c2 1d 1e1 1e2 1f1 1f2 1g1 1g2 2
Dates 1993-11 -
1994-08 -
1995-01 -
1995-03 -
1996-01 -
1997-01 1997-01 -
1997-03 -
1997-12 -
1999-01 -
1999-03 -
2003-11 -
Corner steps 4 5
End handrails Sideways corner stanchions End-facing corner stanchions, angled top rail section
Inverter cabinet panels (left front / right rear) Small, nearly square raised section Wider raised section Taller raised section (approx. 1/2 panel height)
Inverter cabinet panels (right front / left rear) 22" wide panel 19" wide panel; adjacent air intakes moved leftward with narrower panel
Fuel tank Sloped top, rounded top edge, side sheets inside end sheets, 22' 10" over end sheets Flatter top, sharper top edge, side sheets overlap end sheets, approx. 22' 8" over side sheets, seam below top edge Seam above top edge
Phase 1a 1b 1c1 1c2 1d 1e1 1e2 1f1 1f2 1g1 1g2 2
Air pipe behind right air reservoir Jointed Curved
Fuel cutoff switch housing In walkway side frame Wide, below side frame Narrow, below side frame
ECAFBElectrical cabinet air filter box, located on the left behind the cab
Horizontal Vertical
Generator room doors 2 doors (22" + 20") flush with hood 2 doors in raised housing 3 doors (14" x 3) in raised housing
HTCR trucks Version 1 (see HTCR truck details below) Version 2a Version 2b Version 2c
Walkway supports on blower duct 33" apart 37" apart
Phase 1a 1b 1c1 1c2 1d 1e1 1e2 1f1 1f2 1g1 1g2 2
Cable box under blower duct Roughly centered with blower duct Moved approx. 4" rearward
Radiator intakes Vertical Canted
Long hood Rounded top and end corners Angled top and end corners
Long hood end Handbrake near end, louvered panels on left Handbrake moved forward, louvered panels on right
Hood below right-side inertial air intake Flat Angled section extended outward
Dynamic brake intakes Grid, approx. 3 1/8" vertical spacing Vertical bars, approx. 4 1/4" vertical spacing
Phase 1a 1b 1c1 1c2 1d 1e1 1e2 1f1 1f2 1g1 1g2 2

HTCR truck details

The HTCR truck was a radial-steering bolsterless design, aimed at reducing weight transfer under high tractive effort and reducing wheel wear on curves. The primary suspension consisted of springs above the axles and the secondary suspension consisted of stacks of rubber pads between the truck and the locomotive underframe, with a traction pin centering the truck frame with the locomotive underframe.

Rather than being rigidly anchored by the truck frame, the axles were connected to traction rods, which were in turn connected either to the truck frame (middle axle) or to a steering beam and inter-axle link assembly (outer axles). Yaw dampers were connected between the steering links and the truck frame to prevent oscillations at higher speeds. (The inter-axle steering link was removed from later versions of the HTCR after it was discovered the truck performed similarly without it).

The first version of the HTCR truck used a solid bearing adapter design that fully surrounded the axle. The shock absorbers were located directly over the axle ends on the left front / right rear outer axles and both sides of the middle axle, and the bearing adapters were retained by a bracket that extended over the end of the axle from the truck frame.

Starting in 1997, the truck design was revised (what I've called version "2a") to use a split bearing adapter, with a removable section on the bottom that allowed a wheelset to be removed while leaving the bearing adapter in place. The shock absorbers remained on the same axles but were offset from the axle centerline, and the bearing adapters were held in place by chains extending to the truck frame.

In late 1997, the yaw damper bracket was slightly revised (version "2b") with the end attached to the truck frame rotated by 90 degrees. On phase 2 units, the yaw damper bracket on the truck frame was changed from a narrow metal bar to a wider metal plate (version "2c").

A slightly different version of the truck known as the HTCR-II was used concurrently on the SD80MAC and SD90MAC. It was very similar but used an even wheelbase (82" between the axles rather than 80" outboard and 84" inboard) and a wheel diameter of 45" rather than 42".

On the subsequent SD70ACe and SD70M-2, a further revision of the HTCR was introduced (the HTCR-4), along with a simpler, non-steering version with a shorter wheelbase (the HTSC). Externally, both designs moved the shock struts to the outer axles and used a single chain (rather than two) between the bearing adapters and truck frame.

Other details


Biel, Charles. (2020). EMD SD70MAC. Retrieved December 2020 from

Electro-Motive Division. (1992). Loco Relationships, SD70MAC, Part no. L19885. [Drawing].

Electro-Motive Division. (1995). SD70M Southern Pacific Railroad Service Parts Catalog No. 702.

Foster, Gerald. (1996). A Field Guide to Trains. New York, NY: Houghton Mifflin.

Slim, W. C. (2003). HTCR trucks. Retrieved December 2020 from

Unofficial EMD Homepage. (2008). SD70MAC Order Numbers. Retrieved November 2020 from