This is hidden at one of two old-time railroad bridges that were built at the early part of the last century. Both bridges had lift systems that allowed a section to be raised for steamboat traffic. These lift systems were only lifted a couple of times because the bridges, one here across the Missouri and one near Fairview across the Yellowstone, made riverboat cargo hauling obsolete. At times, both train bridges were used for both cars and trains. This bridge is still used by trains but the road decking that allowed cars to cross has been removed. I remember going across this bridge in a vehicle when I was a teenager. It was scary because you relied on a light system to warn of a coming train but the system was not without problems. So you usually raced across, hoping a train wouldn't suddenly appear.
The cache is hidden in a cluster of objects. Parking is close by across the road. Stay off the tracks and the bridge because this is still a working track.