Abstract: An automatic locking system for different sized containers on flat cars or trailers. The locks at the corner of the loading space are not retractable but have automatic locking mechanism that holds the container down. The locks in the center of the loading space are retractable and positioned to lock the adjacent ends of shorter containers and will be automatically retracted into the holder of the locks, when a longer container is lowered down over the locks. The retractable locks are retracted by the weight of the long container when the container presses on a release mechanism placed between the locks on top of the lock-holder. When the long container is unloaded, the retractable center locks automatically are released from the holder and will automatically become locked into a loading position.