Just a wanky way of abbreviating Public Transport Victoria.
N's used to be used back in their earlier years on freights quite a bit, and were quite good at it. Their Head End Power came in useful for powering reefer containers. They were used a bit on Melbourne -Adelaide freights for that reason, and spent many years powering containers on the Tocumwal. And yes, the traction motors were later changed from the little D27's to D77's. It was never really acknowledged, but this upgrade would bumped their weight up to around the same as a G class.
Steve