Remember, the piers also need to support the track, not just look good.
Good point. From a card modeler's biased perspective ... I think cardstock should easily be strong enough. Structures made of heavy card are like little cartons. At that size, they can support a lot of weight, particularly if designed to do so - think of how much weight can be proportionately supported by cardboard shipping boxes or a vertical toilet paper tube.
Another advantage of card: unless you're a "micro-angelo", I think it would be challenging to top the realism of good printed graphics. It looks like whoever does ScaleScene's graphics has an artist's hand.
Then again, here in this forum I often see photographic evidence that many model RRers are great scale painters, so I guess it really depends on personal skills and preferences, how much time/money you want to spend vs. results obtained, how much you enjoy scenery/environment modeling as part of scale RRing, etc.
Nolatron, please post your results, whatever medium you use!