Go with a large cross section round tubing for the beam material, this will hold up better than rectangular tubing for twisting forces from wheel loads.

They twisted after three years of abuse, cambering the wheels a bit (see the teardown video). I operate my equipment hard like a cat D9, so if I went back to walking beams I would use a generous cross section round tubing to take the torsional stresses. As the saying goes, when in doubt build it stout (and then some for tracklayers)!