Couple years ago - they had big plans to replace all our local engines with slug sets. Problem was we have a lot of places where we can’t run around the train, so we have to split 2 engines and go (at least part of the journey) push-pull.
Later on, they had the same idea with replacing all our sets of two 4 axles with a single 6, but the above problems didn’t magically go away.
For a time CSX was using two 4 axles on some of their locals - one on either end of the train and it would permit them to serve customers on a track segment no matter which way the customes switch opened onto the Main Track. Some time thereafter the powers that be came down with the order that all locomotives had to be a part of the locomotive consist at the head end of the train - and the local’s over the road efficiency decreased.
The Bone Valley of Florida at one time was the home of numerous mother-slug power sets. When CSX took delivery of their most recent purchase of EMD locomotives - those 6 axles were assigned to the Bone Valley to keep them in a confined area so their maintenance requirements could be determined and only one service location would have to be trained to service them.
My guess - purely uninformed - is that the 4 axles will get more money at auction than the six axles. So they are trying to replace what they can to sell what will get the most.
After close to a year’s experience in the Bone Valley, those EMD’s appear to have been released to work all over the system. I have no idea what the Bone Valley is currently using for power to keep the phosphate moving.