The FN-class cars were a special "well hole" car. It was essentially a flat car with a large rectangular hole in its top. The hole measured about 26 feet in length. They were built from 1902 through 1916 (road numbers 425001 through 425037).
These cars were 38 feet long and 10 feet wide. They weighed 43,500 pounds and could carry a load up to 140,000 pounds.