The short south from my memory has ruleing grades off 1:66 on the down (Exeter Bank) and 1:66 on the up (Wingello Bank).
Personally I wouldn't bother with most the original alignments on the southern line north or south of Junee .

In the past I got to drive the southern line as far as Junee and had plenty of road trips in between on the Hume Hwy.
Had plenty of road trips over grades on the Hwy such as the one at the south end of Bredalbin Plains , to run trains over a similar alignment and grade is worth the pain to go sailing over the top of the range and on past Gunning . Imagine not having to snake down the range into places like Fish River and Gunning , imagine a line speed for loco hauled of 115 virtually everywhere .
That "Main Southern Line " is dark ages crap virtually everywhere . You must remember even going up the southern highlands to Mittagong on the Hume . Yes there are several very tall and expensive concrete bridges over significant gorges  , it's the lack of things like these that chain rail alignments to the garbage they are today - by todays standards .
Aside from Catherin Hill I can't think of one significant grade on the "down Hume" in this area that would be an issue for pass and freight trains with adequate power .
Spaniards isn't so bad on the road and doesn't force you through Mauldon bends or the "balloon loop" after Picton .
I could go on but you get the point .
I believe the goal for rail freight in the future is to get there sooner and to offer an attractive price .
Blind Freddy knows the only reason road can do this is because its infrastructure is so so much better than rails .
Its been said many times before , and by me as well , imagine how poor road freight performance would be if it had to run over the Hume as it was in 1970 . AFAIK there has been little or no improvements alignment wise to the main southern line since something like 1932 ? And this is the exact reason why rail fails dismally compared to road .
And to the XPT experience , I have never worked on them but had plenty of cab rides down there , in a few various seats …
From what I'm told they didn't make serious inroads into running times compared to the loco hauled expresses . It would have been remarkably obvious to the people operating them that the section times weren't often that much better . It's not rocket science . Put higher performance trains on a twisty windy track and expect good running times ? Lunacy .
The X may have increased Nev's electile capacity , and cost the state of NSW a bomb , but it was still way cheaper than paying for major rail realignments .

In my time as fireman/driver the term of ruling grade was deemed to be what applied between the primary depot/yard points, that applied to both down and up services, which each had a different grade ruling. On the Short North, Light type Garratts, hauled 575 Tons West Ryde to Hornsby, from there to BMD were able to take 615tons. Heavy Garratts took 605 & 650 tons respectively. Pre electrification of Cowan that grade was the ruling one for the whole Short North, but the steam bankers allowed for a through load of 915 tons. Post electrification those loads still applied however an increase was provided for the heavy types, and would be assisted by 46cl up Cowan. Loads were lifted for the heavy garratts to 1130 tons late in the steam days. That provided for a double 46cl load ex Gosford on the up, but very often taxed the garratt through Kotara when the train departed from BMD yard.

The XPT when it was part of the Divisional conference discussions both before and after introduction had the big aspect of not the starting speeds but the overall speed between points.  Using the down on Ardglen as an example and then the Moonbi's in summer the DEB's had to be nursed up the hills, even though they were able to run the TT's as easily as the X's the big problem though was they would readily overheat, wise drivers nursed them up the hills by juggling the throttles and only at full notch for short periods, they lost some time going up the hills but picked them up on the other sections. That was the only way they would run on time.  The X's had basically too much power as you had to do the same with the throttle, by juggling it up and down as you would easily go over the speed up hill (all grades I'm talking of were 1:40) and that meant dropping the throttle back to go under the speed and then up again.

We had around 2 spots between WCK and BMD where we could get close to the 160Km/h but only once CTC came into operation, and they were at Braefield & Parkville the distance from Scone to Togar was not sufficient to get much over 140K's.  In fact many of the old Steam salaried drivers say they could run a similar passenger train with a nanny or pig at the lower speeds but at around the same TT running.

As I have said, the primary objective for my posts and reasoning is not for some great new shiny HST train that needs the track all to itself is to have lines that are rebuilt for the 2000's and not running primarily on 1900's alignments. I have a feeling that we both agree in principle on that but its how it can be achieved is the problem.

As for the XPT I watched a show a couple of nights back on its introduction in England and how the cab was set up, very very different to our version, and the X's successors were also very much responsible for reinvigorating the British rail passenger system and bringing people back to rail. That was Nifties and the NSW railways desires that ran slightly above the desire to eliminate rural services, but it did not really bring the goodies to the state that were advertised/promoted owing to its basic inefficiencies and not really suited to the NSW rail lines, then and still.

As a driver, I found them good to work on, especially as the diagram at WCK had us primarily on step on/off working on the running, where we were round tripping WCK - Maitland - WCK, with a good break at Maitland, the railways wanted us to run to BMD and return but there was not enough time between the arrival and departure times for us to work the round trip as it would inevitably mean the down service would be delayed by a late running up service which was often the case.