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The SD’s were definitely considered western units. The ones equipped with dynamic brakes had a M under the cabs. Once in awhile we would get these units in Toronto. They had a very smooth ride. CN actually had the largest fleet of SD40′s. 5000-5240
The first eight SD40s 5000-5007 (with dynamic brakes) were western lines units even though I had the 5006 out of Mac Yard on 303 on July 2, 1968. The next 100 or so units were primarily used in the east. The 5100s with dynamic brakes were western units used in unit coal train service. The M under the cabs had nothing to do with dynamic brakes. The six axle units, both GM and MLW, were notorious for ripping up the track. These units were classed as “designated units” and were subject to additional speed reductions of 5 to 10 mph below freight train speeds on many curves. The M stood for “modified” trucks which permitted the units to run at regular freight train speeds.
Interesting. I always thought the “M” stood for “Mountain” for units assigned there. I also recall all the “DU” speed restrictions in the timetable; the last one I ahve being from 1990; all DU speed restrictions were on the Bala, Newmarket and and north of that, and there’s a comment in the back that they were in the process of modifying the last DU’s 2000-2043 to remove all DU speeds.
Great comments, guys. Given the sharp curvatures of northern Ontario, B.C., P.Q., Maritimes etc., always wondered why the Canadian roads – especially ACR, ONR, BCR – bought the long wheelbase trucked units ( the SD’s, M630′s, M636′s) ! Those long WB trucks must be very tough on the track gauge viz “notorious for ripping up the track”. Is a six axle unit’s pulling power really that much superior to a four axle unit to warrant the costs of the extra track maintenance (to keep the track ‘in gauge’)? sdfourty
Yes, the 6 axle units really could out pull their 4 axle sisters. For example from Walkley Yard to Capreol a GP40 was rated for 4000 tons while an SD40 was rated for 6000 tons. Given that it has been almost 30 years since the last 4 axle freight units were built (some GP60s for ATSF and UP back in the early 1990s), the results are clear. With radial and steerable truck improvements (never really happened with the 4 wheel versions) increased horsepower to 44/4500 hp became possible without totally destroying the track. And lets be honest, the SD’s gave us a good ride!!