I suppose I will be "shot at dawn for writing this next piece...........................sigh
I watched them changing sleepers both during the actual regauging and latter when they were fixing mud holes and the "side insertation of sleepers" did NOT damage the track base in any of many instances I saw. In most cases I was allowed to get a good look at the track base. One thing needs to be remembered, the West line is the old BG and in the old days this was maintained to a very high standard and had something like 250 to 300 mm of ballast under the sleepers.
There was quite a few base failures on the SG track both north and south of Seymour, but this can be directly traced to the initial poor construction of the line.
The difference in track quality is significant either side of the river. I'm surprised the geology changes so much.
Its probably not the geology, the East line, ie the old SG from Melbourne to Albury was installed VERY cheaply and the has had and stills does have serious drainage problems, ARTC has put massive efforts in trying to rectify the situation, but the track REALLY needs rebuilding from the base upwards.
As for the West line VLine effectively abandoned the line at least 20 years ago, the result being completely clogged ballast, rails with significant vertical bends in them making the ride quite poor, lastly sleepers in a pathetic state something like only 1 in 20 to 30 being any good at all. ARTC again has put a lot of work into this track but so far they have only done around 1/3rd of the total distance between Seymour and Albury, leaving plenty of scope for improvement.