It's an interesting question, given that the Burlington's D-4's and the C&S's B-4R and B-4S consolidations were very close in size, weight, and driver diameters. The C&S's B-4R's (600 series) were a little lighter than the D Bills and their B-4S's (500 series) were a little heavier.
My guess would be that the C&S engines simply were in better shape, having been rebuilt in many cases with new valve gear, cylinders, and other upgrades during the 1920s. The D-4's, unlike many other classes of engines on the Q, never were. So it made sense for the Burlington to use the more modern engines if they could.
However, that begs the question of why the Q never chose to modernize its consolidations while the C&S did. On the Burlington, consolidations as freight engines were pretty quickly displaced from mainline service by heavier and faster power during a period of very rapidly evolving engine design. The Q's relatively flat topography allowed smaller, faster engines that had been bumped from mainline service—prairies, ten-wheelers, even Atlantics, to take over most branchlines, except in a few locations where steeper grades made that impractical. So by the 1920s, the D Bills were increasingly restricted to switching and transfer service. With the extensive and successful rebuilding of prairies into 6-wheel switchers, the consolidations had even less jusification and presumably were retired as they wore out, or their flues did, or as traffic declined during the Great Depression.
Over on the C&S, where ruling grades were steeper, consolidations remained the mainstay of branchline operation from the 1920s through the 1950s (and on the Climax branch, through 1962). So it made sense to rebuild and modernize these engines.
It may be the case, as Hol said, that the leased B-4's sometimes saw branchline service on the Burlington. But my guess is they were mostly used as switchers and transfer engines, as the D-4s had been during their last days.
Jonathan
---In CBQ@yahoogroups.com, <sholding@...> wrote :
So I wonder WHY the CB&Q went and got rid of all the 2-8-0 D Bills only to lease the same wheel arrangement from the C&S….