The railroads typically did this when they expected that they might double track the line. They would then need to have the wider piers to support the wider spans. By putting in the wider piers when the bridge was built it saved on the expense of doing it later.
Anyone know why the concrete supports are so much wider than the bridge itself?
I think you're spot-on Geoff
If you look at Wayne's #4 pic it's darn near the same image.