This is an old railroad bridge across the Yellowstone River that has been converted to automobile traffic for access to the Terry Badlands in eastern Montana. They added a guardrail and put down some metal screen strips that hopefully line up with your tires, but basically you just drive across the railroad ties. Both the ties and the metal screen looked as if they’d not been seriously maintained in some time. At the local BLM office they noted that it was a bit rickety and intimidated people at times but said they’d not lost anyone on it yet. After two trips across in my van and walking out and back once, I am happy to report their claim still stands.