This will not be an answer to your question as much as an extension of it.
I see the same WEPX coal trains on the CP C&M subdivision, where they complete their trip to the Wepco electric generation plant south of Milwaukee, WI. I first noticed these BNSF coal trains on the CP C&M in early January 2009 as CP trains 807 (westbound loads) and 808 (eastbound empties). What surprised me then was seeing that they did not have the rear DPU locomotives, given that all the BNSF coal trains I had seen farther west in rural Illinois and Iowa and Nebraska did have rear DPUs. I figured this meant that the DPUs were being removed from incoming trains somewhere before the trains were handed off to CP, but I did not know where or why. And after hearing one of the engineers mention that his train 807 was 19,000 tons, I understood why they sounded heavy, with the lead locomotives obviously laboring to reach a modest speed even on the rather flat terrain here.
Then sometime in the late spring or early summer of 2009 I noticed that these BNSF WEPX trains started to have rear locomotives when on the CP. At times I have seen an eastbound empty with a single lead unit and two rear units, but I do not consistently enough see the trains on consecutive days to know whether this is exactly the previous day's loaded train returning in reverse (rather than having gone around a loop) or is just a more random feature of the consist. Sometimes I hear the dispatcher telling the engineer whether to "put away the locomotives" when arriving at the Wepco plant and other times they are just instructed to leave the entire train on a particular track at the plant, so I am not sure that the trains necessarily leave with precisely the same locomotive consist they had upon arrival.
Incidentally, sometime in the last few months the numbers of these Wepco-bound BNSF trains operating on CP appear to have changed to 813 (westbound) and 812 (eastbound).
