Q: In the May 2025 Ask MR, the answer to a question about early diesel power for the Milwaukee, Racine & Troy included this statement: “For road power, the MR&T would most likely turn to four-axle units, such as the Alco RS1 through RS3 and EMD GP7 and GP9. It’s possible the MR&T could have had a few EMD SD7 and SD9 diesels for branch line duty.” Why would six-axle SD units be appropriate for branch lines? My understanding was that SD units were significantly heavier than the comparable Geep, and that Special Duty referred primarily to the fact that heavier units with six motors were better suited for low-speed, heavy-tonnage assignments. — Michael Paster
A: Your initial instincts are correct, Michael. In a Burlington Northern Dakota Division employee timetable from Oct. 30, 1988, there’s a numbered list below each subdivision. Item two covered “Bridge, Engine and Heavy Car Restrictions”. I checked the branch lines served out of Crookston, Minn., my hometown, and all have the following: “Locomotives in Groups G, H, and I not permitted.”
Group G was ex-Chicago, Burlington & Quincy Electro-Motive Division SD9s. Group H was former Great Northern EMD SD9s. Group I covered General Electric U30Cs and C30-7s, along with EMD SD38-2, SD40, SD40-2, SD40-2B, and SD60 diesels.
However, on full-size railroads there are sometimes exceptions to the rule. For example, Milwaukee Road purchased EMD SD7 diesels, in part, for use on branch lines that had light rail and minimal (if any) ballast. To keep the weight of the locomotives down, Milwaukee Road ordered its SD7s with a single 1,200-gallon fuel tank instead of two tanks. Further, the SD design allowed the weight to be spread over six axles instead of four.
In addition to SD7s, Milwaukee Road also had 10 EMD SDL39 diesel locomotives. The six-axle units, built in two groups of five in 1969 and 1972, were also designed for branchline duty, replacing older Alco six-axle units like the RSC2 and RSD5. Weight-saving features on the SDL39 included a 12-cylinder diesel engine and export-style truck sideframes.
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