These are called Guard rails.
Guard rails are placed parallel to regular rails along areas of restrictive clearance, such as bridges or tunnels. They are used to minimize chances of derailment and ensure that wheels of the trains are aligned in case of derailment. If the train derails, the wheels fall off the regular rails and the train can move pretty far sideways because of which the train could fall off the bridge.
You can see them on regular train tracks too, but only on overpasses or bridges or places where derailment would be quite a possibility. They are placed inside the inner rail, where they engage the back of the flange of the wheel on that side.