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Locomotive Descriptions and Phases - EMD SD70, SD75 series

The SD70 series was introduced in 1992 as an evolution of the SD60 series. It retained the 16-cylinder 710-series engine, uprated from 3,800 to 4,000 horsepower. The biggest difference was the introduction of the radial-steering "HTCR" truck, on which horizontal traction rods connected to a steering mechanism could slightly alter the orientation of the axles, keeping them in line with curved track.

Unlike the concurrent SD70MAC, the DC-powered SD70 series used two separate traction motor blowers, replacing the single central blower and eliminating the previous air duct behind the cab and along the left walkway. The remainder of the carbody was similar to earlier EMD models, with the dynamic brake intakes and fan immediately behind the cab, a nearly flush exhaust housing near front of the engine compartment, and three radiator fans at the rear of the hood. The shape of the trucks and carbody together gave the SD70 a long, clean-lined appearance.

Both a standard cab (SD70) and a wide-nosed safety cab (SD70M) were available, the latter using the design that appeared on late SD60M production, with two "teardrop" windshields and a tapered nose. In 1996, CN ordered a version with an isolated cab (the SD70I) and a 4,300-horsepower version was ordered by BNSF, CN and Ontario Northland (the SD75M and SD75I). In 1999, Union Pacific placed the largest single domestic locomotive order up to that time for 1,000 SD70M units, later increasing the total to more than 1,400. These units were intended to replace the SD40-2 on UP's roster as a reliable general-service locomotive, and accounted for the vast majority of total SD70M production. Later units introduced flared radiator intakes (housing a split-cooling system for the radiators and turbocharger aftercooler) and a simplified cab with sharper corners.

In 2005 the SD70M was replaced with the SD70M-2, a substantially revised design that met Tier 2 emissions standards.


These phases are of my own making (originally from around 2008 and revised in 2022). As UP units accounted for almost all production after 2000, it's difficult to know if all these details would have equally applied to orders from other railroads.

I've divided sub-phases (2b1 / 2b2 etc.) for variations in the cab and fuel cutoff switch, which alternated between early and late versions across several phases.

Phase 1a 1b 1c 1d 1e 2a 2b1 2b2 2c1 2c2 2d1 2d2 2e1 2e2 2f 2g 2h
Dates 1992-07 -
1993-06 -
1995-04 -
1997-08 -
2000-10 -
2000-11 -
2001-12 2001-12 -
2002-01 2002-01 -
2001-12 -
2002-03 2002-03 -
2003-01 -
2003-04 -
2003-04 -
2004-09 -
Side underframe Flat panel above fuel tank No flat panel above fuel tank; revised traction motor cable routing
Corner steps 4 5
HTCR Trucks Version 1 (see HTCR truck details below) Version 2a (early CN SD75I)
Version 2b
Version 2c (appeared on late Phase 2d1 units)
Short hood bottom access panels Rectangular Angled top corner
Phase 1a 1b 1c 1d 1e 2a 2b1 2b2 2c1 2c2 2d1 2d2 2e1 2e2 2f 2g 2h
Radiator intakes Vertical Flared, 2 screen sections;
smaller bolted panel under right intake
Fewer bolts around radiator and inertial intakes Flared, 4 screen sections
Generator room doors 2 doors (22" + 20") flush with hood 2 doors in raised housing 3 doors (14" x 3) in raised housing
Panels above dynamic brake intake 10 bolts, 2 panels 8 bolts, 1 panel
Panels under inertial air intake 16 bolts 8 bolts
Long hood end Rounded corners and light housing Rounded side edges, angular top edges and light housing Angular side edges, top-mounted grabirons
Phase 1a 1b 1c 1d 1e 2a 2b1 2b2 2c1 2c2 2d1 2d2 2e1 2e2 2f 2g 2h
Fuel cutoff switch housing Either in sideframe or large housing below sideframe Small housing
below sideframe
Large housing Small housing
Dynamic brake intakes Grid, 3" vertical spacing Vertical bars, approx. 4 1/4" vertical spacing
Short hood Version 1 - tapered (narrower at front), rounded corners, 2 upper side bevels, bottom-mounted grabirons
Version 2Not tapered, angular corners, single side bevel,
side panels slope forward, top-mounted grabirons
Version 1 Version 2
Left side handrail stanchions 5 stanchions behind fuel tank 4 stanchions behind fuel tank
Jacking pads Flat metal plate Open
Phase 1a 1b 1c 1d 1e 2a 2b1 2b2 2c1 2c2 2d1 2d2 2e1 2e2 2f 2g 2h

HTCR truck details

The HTCR truck is a radial-steering bolsterless design, aimed at reducing weight transfer under high tractive effort and reducing wheel wear on curves. The primary suspension consists of springs above the axles and the secondary suspension consists 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 are connected to traction rods, which are in turn connected either to the truck frame (middle axle) or to a steering beam and inter-axle link assembly (outer axles). Yaw dampers are 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. In late 2002, 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


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

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

Strack, D. (2015). Union Pacific SD70Ms. Retrieved April 2022 from

Unofficial EMD Homepage. (2007). SD70, SD70I, SD70M Order Numbers. Retrieved April 2022 from

Unofficial EMD Homepage. (2000). SD75M, SD75I Order Numbers. Retrieved April 2022 from