I forgot another advantage a GP39 wold have over a GP38 in some situations.

EMD engines with roots blowers have a bad habit of starting trackside fires. If the engine is used at low throttles, it will eventually build up carbon and oil in the exhaust system, especially if the power assemblies are worn or it isn't allowed to warm up to full operating temperature. When the engine is eventually operated at full throttle, the exhaust passages are heated to high temperatures and the carbon starts breaking loose after about 15-20 minutes. If the carbon pieces are big enough, they will retain enough heat to start a fire. In certain situations, you can also get a stack fire with flames shooting 6 feet into the air. My record is witnessing the SW8 I was running start 29 individual fires within the length of the engine before I gave up counting. THe conductor and I had been told by our office to keep going, but we finally stopped, shut the engine down and called the fire department. They were kind enough to put the fire out that was threatening the rear of the train, and also the headwalls of our small trestle that were burning. I started a few fires this summer with GP15s after pulling full throttle on some short hills. Strangely, some of these fires were after I began backing off the throttle going over the top.

Turbochargers (such as on a GP39) do act somewhat to reduce the chance of starting a fire. The turbine blades will often break up carbon into small enough pieces that they can't retain enough heat by the time they hit the ground. While less likely to start a fire, I have seen first hand a turbocharged GE and EMD puke like a volcano because they were carboned up from extended low throttle service.

You will often see GP38s and other EMDs with roots blowers in mainline trains being moved dead in consist between shops or assignements. While fully capable of adding more horsepower to pull the train, special instructions often restrict them from being under power to assist mainline trains in high fire danger areas.