Locomotive Descriptions and Phases - EMD SD70, SD75 series

CN #5682, a Phase 1d2 SD75I seen in 2009.

The SD70 series was introduced in 1992 as an evolution of the SD60 series. It retained EMD's 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, which replaced the single central blower and eliminated 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.

Phases

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.

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. 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

• Phase 1d and later UP units have an underframe light under the right side cab.
• Phase 2b1 and later UP units lack access panels on engine compartment roof.
• The right rear speed recorder cable was removed partway through UP Phase 2d1 production.

References

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

Slim, W. C. (2003). HTCR trucks. Retrieved December 2020 from http://www.okthepk.ca/dataCprSiding/articles/200307/trucks.htm

Strack, D. (2015). Union Pacific SD70Ms. Retrieved April 2022 from https://utahrails.net/articles/up-sd70m.php

Unofficial EMD Homepage. (2007). SD70, SD70I, SD70M Order Numbers. Retrieved April 2022 from http://www.trainweb.org/emdloco/sd70.htm

Unofficial EMD Homepage. (2000). SD75M, SD75I Order Numbers. Retrieved April 2022 from http://www.trainweb.org/emdloco/sd75m.htm