If it hadn’t been for the acquisition of Conrail, NS would be an all DC railroad. I don’t know why they invest in new DC locos because AC locos are more efficient and offer more tractive effort.
Two reasons one is cost For the price of 3 AC’s you can get 4 DC’s and you also need to have all the parts needed to keep the AC traction systems running. The second in the NS case is Cost they still refuse have the newest power rated at over 4000 hp instead have the computer limit it to that instead of 4400 in the GE or 4300 in the EMD their thinking is we just need to put as many 4000 HP units together as needed to get the train over the road as needed.
IIRC, NS also has an override switch so the engineer can use all 4400 HP when needed for a short duration. Otherwise, like you said 4000HP. Their thinking is that less stress (wear and tear) will mean the units will last longer.
Actually it takes a Trainmaster or Road foreman of the Engineeres to have the password to increase the power on the engines got that directly from a friend of mine that is an engineer for the NS.
NS has made the decision to power their trains sufficient to stay above minimum continuous speed on the ruling grade. Give that there is no big advantage to DC motors. The only place AC does something DC can not is at less than MCS for DC motors. I suspect that MCS is 14-15 MPH.
Mac