While looking thru photos after David's question,I ran across this photo of 3239:

http://southern.railfan.net/images/archive/southern/sd40-2/3239.jpg

The truck under the cab does not look like the standard SD-40-2 HTC truck.  Is it an older design or something Southern was testing?

That is an HTC truck with the casting holes blanked, somehow or for some unknown reason. It looks like possibly they were filled in, or even photoshopped out to me, but that is only speculation. The photo is undated, but obviously post-NS merger era, so, if a modification of some kind, it was a post-NS merger one. I have a 1975 shot of SOU 3239 with standard HTC trucks on both ends and shot her again as NS 3239 in 1994 still with HTC trucks. Other than the lack of casting holes in the sideframe, it appears to be a normal HTC truck. The long hood end truck in the posted photo of 3239 looks normal to me. If this was a test or trial of some sort, the truck sideframe would likely be wired with sensors for instrumentation; there is nothing to indicate that is occurring. There were truck modification trials done by EMD during the 1980's resulting in, among other tests, the "guided axle" trucks used under some of the early EMD AC traction models and the "radial" trucks now common on later EMD 70 series.