Generally, a freight car hauls goods one way and comes back empty. In an effort to make the most out of their railcars, Burlington Northern experimented with boppers, or boxhopper cars, that could carry both boxcar freight and hopper car freight. These aluminum cars were equipped with movable "walking" floors that were meant to shake hopper goods into discharge chutes. The floors were hydraulically powered by a generator located in a power car. In the end, the boppers didn't work out for the BN. Though some were scrapped, Burlington Northern Santa Fe Railroad donated three boppers and a power car to the museum in March 2002.