A Small Mystery
Railroad bridges are generally equipped with guard rails between the two running rails, the function of which is to prevent errant wheels from deviating too far from center in the event of a derailment on the bridge itself, thereby hopefully keeping the cars from tumbling off. At the ends the guardrails taper to a point in the hope of catching and properly guiding a derailed axle if one were to approach the bridge span.
Here is a photo of the BNSF Canyon Diablo bridge in Arizona, The question is, while the left-hand man is equipped with guardrails, the right-hand man is bare. Why? It's 220 feet down to the floor on either side.