1553,Yes, the points you make about Y112 running on the main line are quite valid, it sounds near impossible to run it these days. Perhaps it is time for it to be a regular at say the VGR. True the expense of strengthening may not make it viable but, with the upgrading of the rail from 60 to 80lb soon to commence, it may be able to be incorporated within these works. Bridges may already be strong enough, I am not sure that it has been investigated that closely, probably not at all, at this stage.
Y112 hauling a mainline train from Melbourne to Castlemaine? Hardly realistic. Maybe pre-RFR, but not now. If it was still duel trackage, again, maybe. For starters, the tender only has capacity for 2200 gallons of water. This means that you have to either water on the run from a water gin (not impossible, but it does limit the amount of carriages you can haul, which in turn affects patronage), or stop to water the loco at regular intervals. The second and biggest problem you'd have is that the Y is limited to 60km/h maximum. Try getting V/Line to agree with running that amongst V'Locities - you'd be stuck in one of the loops for hours. Best to let the Y make it's way up light engine at night when it can't disrupt anything.
Strengthening bridges? Why? The VGR is a branchline. Why go to all that expense of strengthening something, when the heavier locos would only use it on odd occasions? Not very realistic at all.
What future would you like to see for Y112? With all the man hours and expense to restore the loco, it seems wasted having it run a local shuttle once a year.