I would say it is the most curved and not the worst permanent way. The goat track that is the Vic NE would be the worst permanent way. The XPT is actually extremely slow in the curvy bits and is beaten by the Xplorer and Endeavour in this regard. When the Xplorer runs to Dubbo it is actually a faster service then the XPT. The new trains will be even better in this regard since they will be faster accelerating and better handling. The new trains will have a higher top service speed to boot of 200km/h, not that it would be possible to get to that speed anywhere in NSW.
When the XPT's were introduced, I was on the NSW Divisional Council at the time and the #4 regional councillor which still had the old Deb set in operation, compare the two, there is none even on the relatively poor track at the time. The reports from those who were working on the first sets, and doing the tests made big inputs into their operations and that was in reference to their pick up in speed as against overall operating speed. From standing start, it did not get going until 2nd notch was opened and the set, and power unit would act in very much a vibrating machine and rock from side to side until a bit of speed was gotten to and the train got into full throttle.
In overall operation, when having to slow for curves and then pick up again, the same slow pick up was found in trying to get to track/timetable operating speed. This was especially so in the days before the overall Time tables were somewhat faster, on some lines the district councillors reported that they needed to brake and start throttling up with the brakes still on to hold the speed, and then to obtain the required exceleration when the brakes were released. It was very much a no no but drivers were bunged if time lost. The union put the case to arbitration and railways the conflict in exceleration verses overall speed and in the areas where there were a lot of curves there was a need for timetable variation and added time for operation.
In the cab of the XPT, you would feel every bump and problem in the track, bad sections had you bouncing up and down along with sideways movements, especially over lines where there were timber sleepers. As the Hunter Valley big track replacement program came into being during the 80's where the whole track was pulled up and all the old sub soil and ballast was removed with the ballast going through size grading machines in the cleaning process, along with concrete sleepers and the heavier 63kg rail, on the completed track work, the whole ride in the cab was smooth and great.
Working the XPT to Armidale it was a different story as the track was very much old style 106lb rail on timber sleepers with many poor ones, and speed limited to 120km/h in certain spots only, it was bouncy and ruff. I rode with a Taree driver on the down daylight service from Gosford to Taree when there was one old section of track north of the tunnel at Gloucester the section was around 3k's in length and had old timber and the 106lb rail not welded, it had a speed limit of 80km/h on it and very rough.
With modern day track, concrete and heavy rail in constant welded in theory it should be better, as for the aspect of what is causing the cracking in the centric pins, I would have little doubt that a cause includes the stresses of the early days, along with the aspect of how many Km's they do on average a day. When one compares them to old loco hauled passenger trains and of old world loco's such as the 42, 421, 44, 442, 49 classes each which would let you know when you hit bad track and why as drivers we had to put track defect forms in at the first manned station/signal box on a carbon copy booklet supplied, I wonder if they still exist, which I would likely say not as I remember Shirley ordering their removal, but they came back after he left.
Each of those diesels had their own tell tale areas of telling you what the track condition is/was like, I know of a fireman who was thrown off his seat on a 44cl when hitting a bad spot as they would have a huge sideways swishing movement on bad holes or the like.
I wonder though as far as track conditions go in these days is a combination of not just the maintenance needed which to there would be little doubt of is the general lack of heavy maintenance overall along with heavier locomotives and trains, likewise the length of them and the higher speeds they operate at does not also have an impact on the tracks, especially where the alignments are still as they were in steam days.
Finally the area that I always found in my time as an engineman was the general lack of spare parts for the locomotive fleet, where even at big depots such as Delec, & BMD where there was large loco allocations where a part had to be despatched in the cab of a locomotive on the first available train heading to another depot needing a part. very often seen at Delec for parts to Lithgow and seen elsewhere.
I am therefore both surprised and not, as to the pins failures owing to age of the fleet and not having spares on hand at Meeks Road for the fleet.