The engine had converted the wheels of the SD-40 to P48 standards, meaning the wheel profile is prototypical. That was good for me, since that looks better and I want it that way, but also since my (only) turnout has flange ways etc that are more P48 than standard O. The loco axle length had also been adjusted to a more narrow and prototypical track gauge. This was not equally good for me, since my track gauge is standard O. Don't ask why. It's just is. This meant my new loco would not run on my track, unless I did something about the wheels, or rather their spacing.
So I have done some experimenting with one of the wheel sets. The conventional O scale gauge is 60 scale inches, which equals 1.25" or 31.75 mm but Proto:48 uses the prototypically accurate 56.5 scale inches which is 1,17" or 29.90 mm. That is not a very large difference, and the loco, as arrived, could actually sit tight on my track. It even powers up, and makes all sorts of sounds. But when I throttle up it immediately derails.
Since we are only talking about 2 mm, I simply separated the wheels on one of the axles by that amount. The axle is a little to short for that and I am not sure that the wheels will stay put in the long run. However, there want be much stress on the wheels on this little layout, and it might be enough to secure the wheels with a little Loctite. Another solution would be to buy new axles from NWSL. Fitting axles seems to be readily available for $1.25 each, so that is might be worth the trouble. The picture below shows separated wheels on the axle uppermost in the picture. You can see that the wheels sit tighter on the lower axle, P48 style.
Another problem is that although the gearbox of the truck is originally done for the longer axle, the original wheels were much wider. With the narrower wheels separated further apart there is a risk that the wheel with the gear slides that far out so that the gears lose contact with each other. To fix that problem I inserted a spacer between the the opposite wheel and the gearbox. The next picture shows the left wheel, with the gears, at its outermost position. It still contacts the rest of the gear assembly, even if it is a close call.
The non-geared wheel is electrically isolated from the axle, and uses a sprung pin sitting in the gear box to make electrical contact with the motor. You can see the pin in the lower right of this photo.
If the isolated wheel slides to far out from the gearbox, the pin will no longer contact the back of the wheel, and the electrical pickup is broken. To avoid that I put a spacer on the geared wheel side also. So now the isolated wheel cannot go any further out than this picture shows.
Even if I decide to buy new axles, I will probably start by just separating the wheels on the existing axles. Just to be able to see the loco run.