Thanks msh.
You might notice that my benchwork is also built out of balsa. This makes for super lightweight modules!!!
Balsa is actually quite strong. That's why aeromodellers use it!
Obviously if you king hit it or sit on your benchwork you will smash it, but how often do we sit on our benchwork?
My thoughts are - why do we use such H-E-A-V-Y material to build our benchwork when the only loads we intend to place on it will be no heavier than an N-scale train? (I use light weight foam for scenery).
As far as "above" the benchwork goes, well, my model bridge is never going to carry a load any greater than a few N-scale plastic vehicles so, even in balsa, it is "over-engineered"
I will cut and lay a styrene plastic road base/shoulder across the top of the balsa and use plastic Arnold highway railings to complete the bridge.