For the P42....One provides hotel power, reducing its tractive effort. Don't recall exactly how much offhand. The rest are purely tractive.
engine output of 4,250 horsepower (3,170 kW) at 1047 rpm, or 3,550 horsepower (2,650 kW) when running in HEP mode (900 rpm) with a 0 kW HEP load. Traction horsepower in HEP mode decreases to a bare minimum of 2,525 horsepower (1,880 kW) when providing the full 800 kW HEP load to the train.
Why do such engines run in idle power. I would have thought it would be useful to either have it contributing traction power if that can be justified, or otherwise be towed dead.The third unit is most likely a deadhead move. It is just being moved from one location to another. Most likely not working just along for the ride in idle power.