As for the track, if there is any tiny bump in it, the front truck will pop off, and this is how I found an issue in my track, as the truck would bump off, and then derail on the next turout. The reason is that by using a 6 axle diesel on 19.5" radius curves (in my case), there was tremendous pressure on the wheel flanges, and they could not ride up off the track momentarily and come back down still on the track.
The SD40-2's are starting to get used as yard switchers and helpers in very large operations, like North Platte. The GP series locos would be great for switching though, especially as a 2 unit MU. If you want widecabs, though, you are pretty much out of luck, other than the awkward-looking GP60M.