When you think about it, it makes a lot of sense to rebuild a gp40-2 into a gp38. You rebuild the engine and remove the turbo in the process. They would have to put in new pistons to raise the compression back up to compensate for the lack of a turbo, but the whole thing would be a lot less expensive than selling a relatively worthless gp40-2 and then buying a brand new gp38-2 or something similar for switching and branchline work. Emd came out with the gp38-2 for that work, but the class one railroads have a bunch of obsolete turbo charged power that isn't powerful enough for mainline work anymore, but unnecessary for branchline or local switching. The final advantage to the rebuilds is that naturally aspirated diesel engines are more reliable than turbo charged diesel engines.