Peter I think you sell the NRs short .All I said was that the 81 and 82 classes were more suitable for slow and heavy trains..
Earlier traction motor issues were actually pinion issues , not motors melting .
Aside from this I always though their adhesion issues were cause by their hardly hi tech bogie design .
I've told you of these times before .
Please explain how I got 1999 tonnes up Demondrille with one in admittedly good conditions , 1300 amps at 13 km/h down to about 8 over the top . Never missed a beat . Also got a bit more up Jerrawa with one in the dry .
One you wouldn't know about was the time we set sail out of Coot West one evening on PS6 . This one had part off a steel train attached out of Parks and weighed in at 4200 tonnes - a full load . As circumstances would have it the controller in Junee kept us going through Yass behind a wheatie , in the wet , that was leaking grain onto the rail heads .
Crawled up the Cullerin ranges as you'd imagine at 14-16 KM/H , never looked like slipping to a stand .
Unless things have changed full loads on the up south north of Junee were 1780 for Gs and 1840 for 81s , I think NRs were 2100 . I think you could easily substitute a pair of NRs on a wheatie that two 81s typically haul .
I think the best thing you could do was fit them with the later flexi curve bogies if suitable . I reckon that if you can get them to hold their feet you have a better chance of maintaining a speed at which the traction motors won't overheat .
Also , no spoke about it much but 81s didn't tend to survive too long tripping steel rakes out of Cringilla . Generally about a week or two and they would fry traction motors . They are good things but even the best can be beaten .
The NRs are model Cv40-9i. The diesel engine will derate (the "v" for variable horsepower) down to 2650 HP.
Two 81s are theoretically allowed 39 NGPF/NGKF which are I think 82 tonnes fully loaded on the 1 in 66 ruling grades on the South, so 3198 tonnes. From memory these trains slow to around 19 km/h on the sharp curve just before the lower bridge, but increase speed until the curve at the turn to the final climb to the top bridge, where they slow again through the sharp curve.
Currently, PN allow up to 42 NGPF/NGKF on these trains. The 82s have slightly better motors and better controls, and two 82s or one 82 and one 81 make better time than two 81s.
On a sustained climb when a pair of NRs couldn't rely on maintainig a higher speed, they may well take longer on the climb than two 81s if they had to fully derate to protect the motors.
Since the NRs aren't any more environmentally friendly than an 81 or 82, and spares might become a problem in future compared to the EMDs because of the larger number of similar units in the USA (particularly the traction motors), I'd expect the 81s to stick around. The V/line N class had their D43 motors replaced by D78s simply because D78s were available rebuilt ex the USA at a much lower cost. I have no idea if any other loco used the GE 793.....
While you could probably fit Flexicurve bogies, you couldn't use the existing traction motors which are not suitable for the "dogbone" suspension links used to allow the radial steering of the axles. You could re-use the armatures in a new casing, but that would be expensive and you end up with a locomotive that is no more capable.
The 81s have been hauling the grain for forty years or so, and unless their maintenance costs increase, they will keep doing so until replaced by AC locomotives, maybe 93 class when coal traffic declines.