Bridges Bridges are needed to cross over valleys, rivers, washes, roads, railroad tracks, and other obstacles that will be found along the route.  Bridges that do not have any effect on water flow are reasonably simple to construct so as to not allow the passage of humans under or over each, it is the bridges that must pass over railroads, roads, and moving water that present a challenge to the design of the BorderTransportationSystem.  Additionally, there materials used should be immune to weather and people damage. Here are a few of the engineering and material challenges that must be considered when constructing bridges along the BorderTransportationSystem. Index of ... Coatings to resist rust and bird poo How to prevent unauthorized climbers How will heat effect the materials Long Bridge / Elevated Track Construction River Crossing Security