Guard Rails

 
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In railroad use, guard rails are placed parallel to regular running rail along areas of restrictive clearance, such as a bridge, trestle, or tunnel. These have the effect of keeping the wheels of rolling stock in alignment in case of derailment. It also helps to minimize damage to the structure and allow easier post-accident cleanup.
On sharp curves, guard rails may be placed inside the inner rail, where they engage the back of the flange of the wheel on that side.