You've gotten good answers so far. Skeleton log car (or wagon), around the turn of the 20th Century. The rails were used to keep the logs from rolling, 6 logs were loaded per car, in a 3-2-1 pyramid. Narrow gauge was common.
These cars would have been used by logging companies to get logs down out of the wilderness to where another railroad would pick them up (usually transloading the logs onto a standard flat lumber car with a rack). Less often, they would take logs right to the sawmill if it was a company-owned one.