The issue with the NCR isn't grades it's the deliberate avoidance of building bridges and tunnels - low cost approach .
Also btw if the grades got straighter steeper the coal trains would be powered to suit , don't think you're going to be seeing a pair of 82s dry retching their way from Tahmoor to Moss Vale for much longer .
IRR will not turn the NCR into a grave yard because the east coast has a Sydney on it and Sydney , ok Newcastle Sydney Wollongong , has the largest consumer market on the island called Australia . Same broken record , NOT economic to shuttle Sydney traffic from Brisbane or Melbourne back and forth to Parkes .
And again , how many trains a day/week/year only convey traffic between Melbourne and Brisbane exclusively ? Bugger all . Most Aurison and PN MB/BM trains interchange at Glenlee or Chullora . A more accurate handle would often be MSB/BSM .
IRR . If it were an exclusive speedway how many superfreighters are going to be able to run flat out for a dozen hours at a time ? Are we going to have passing lanes you scoot through doing 115 points and all ? This is going to be a single line isn't it and people want to run slow grain trains etc along it to bulk up the number of trains paying access fees to use it yes ? Then I suppose some government body will want to put short light higher speed pass trains on very likely with pass priority yes ? And I gather these will have to have station stops so country voting citizens get some tax bang for buck ? Fast hmm , short transit times hmm - doesn't necessarily work in other places hosting flattish straightish rail corridors .
Lets look at some examples like say Blacktown to Kingswood , fast you think ? Nope , try again because there are too many stations with things called EMUs clogging the system up . Same Islington Jct to Maitland (DMUs) same Glenfield to Campbelltown same Ourimbah (sp?) to Morisset . These really aren't so different to Murrays Flat or Bredalbin Plains alignment wise , the major advantage is very few MU trains and no stations to stop at .
So , build an IRR and run trains of varying performance capabilities over it and you get pretty much what you have now on any single line . Straight and flat is not fast when following other trains or looking at red lights in a loop . Straight and flat can be fast and timely when you don't get blocked for any reason anywhere but a small handful of trains per day isn't gonna pay for lots of hundreds of km of brand new high standard railway are they ? Take to the grave , XPTs were made to be fast-ER because there was the political will , read mileage , in making them so . Funnily enough this was the same with the birth of the "superfreighter" trains like the old TNT Contrans 3177 and 3196 , and 3175/3194 to from Trackfast . Drivers were told to whip these overpowered short light trains and no one gave a rats about fuel provided it didn't run out . These trains driven with gusto easily outgunned and consistently outran the "crack" interstate pass trains of the day - no doubt by an embarrassing margin . The control boards were told to part the waters for them and go hard and fast they did .
The major change is the culture and the lost will to make the fast freighters - fast . If it still existed the freighters would be faster than they are but so much has changed .
Roll forward to now . A big superfreighter is 1500 + metres long not 670 and weights 4000T not 2000 . It isn't run by the state government it's run by a private mob who cops whatever pass priority crap the government track provider and controller hands out .
Really at the end of the day the way to make a rail corridor work is to have it so the slowest trains can run more often at their maximum speeds . And , if the slowest trains are stopping pass services then NOTHING behind will get a run - including the X .