Here's a zoomed piece of this image from Wikipedia
That's a close-up of a tower holding a railroad bridge. A and B are two of the four columns in the tower's corners. I assume that they are placed symmetrically, so arrow B points onto the typical "outerwards" surface and arrow A points onto typical "inwards" surface.
Note that where arrow B points the surface is solid and where arrow A points there's a lattice instead. Note the numerous rivets alongside each beam - the beams look like they are box girders assembled such that each of them has three solid surfaces and one lattice surface.
What would be the reasoning for having a lattice on one of the surfaces only?