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The Railwire is not your personal army.
So when I saw it I was caught up by the B unit sandwiched between the SDs. I have some SOU pics from the same time period with random F3/7 As and Bs mixed in with all kinds of current gen power. I am working toward having a few of these”oddball” consists on the layout when it gets to that point.
Looks like it could be a holdover from a time when 4 F-units were assigned to such a train and there was a requirement for 16 axles worth of power.Two of the newer SD locos might have been sufficient to power that train, but the extra F-unit was added to bring the axle count up to 16.
The irony of adding the B unit is that it actually reduces the tonnage the trio can pull up a grade... two SD40's are able to lug more tonnage than two SD40's and an F unit. Going back to an excellent three part R&R article from the 90's about the adventures of an EMD field technician overseeing the delivery, set up, and training of staff on new locomotives, there was an anecdote about an SD45 delivery to EL where the EMD man was accompanying the new units on their first trips. One experience was, as I recall, two SD45's and an F unit climbing Gulf Summit with full tonnage. Train stalled after some degree of struggle, and the EMD man had the crew isolate the F unit and try to get the train started with just the two SD45's. They walked up the grade with that train with little or no trouble to the surprise of the crew. The issue was the pairing of the SD45's IDAC wheelslip control vs whatever the older system the F unit had. The IDAC allowed an ever-so-slight amount of controlled slippage while the F unit, not able to perform in that manner, would start to slip wildly and then drop load to compensate, then load up and start to slip again, and repeat. Isolating the F allowed the IDAC to do it's thing unimpeded.I really wish I could reference the article directly (those issues are buried somewhere and I've not seen them in years so I may be recalling some details inaccurately) but that example of the superiority of the newer wheelslip control has stuck with me ever since.