1. A railway flat car comprisinG a deck, a plurality of container supports each having a base on the deck, a container having lower corners supported on the container supports, each of said lower corners having an opening therein with an arcuate edge defining the lower portion of the opening, each container support having a generally horizontal seat to support the associated lower corner of the container and a pair of connected walls arranged in a right angular relation and extending upwardly from the seat to restrain the container against horizontal movement, a releasable locking member for each container support having an upper end extending through an opening in one of the walls, said upper end including a protuberance extending inwardly from said wall opening and adapted to extend into the adjacent container opening for restraining the container against removal from the associated support at a lifting force below a predetermined minimum lifting force, means urging the protuberance toward the container opening, said protuberance having an upwardly inclined lower cam surface for contacting said arcuate edge defining the lower portion of the associated container opening, said lower cam surface being generally arcuate in cross section to fit in a generally nested relation against said arcuate edge upon upward movement of the container, the upward movement of the container being restrained by said locking member until a predetermined minimum upward force is exceeded whereupon said locking member is urged outwardly by contact of said arcuate lower cam surface with the arcuate edge of the container corner defining the container opening, and means on the associated corner support adjacent the lower end of said locking member hinging the locking member for a limited rocking movement of the protuberance in a direction generally parallel to the inner surface of the associated wall.

5. A railway flat car comprising a deck, a plurality of container supports on the deck, each container support having a generally horizontal seat and a pair of connected walls arranged in a right angular relation and extending upwardly from the seat for restraining a container against horizontal movement, an upwardly extending releasable locking member mounted on each container support adjacent one of the walls, the upper portion of said locking member including a protuberance extending inwardly from one of said connected walls and adapted to extend into an opening in the container for restraining the container against removal from the associated support at a lifting force below a predetermined minimum lifting force, means on the container support mounting the locking member and protuberance for a generally pivotal movement in a direction generally perpendicularly to the inner surface of said adjacent one wall, said means further mounting the locking member for a limited rocking movement in a direction parallel to the inner surface of said adjacent one wall to permit the protuberance to easily fit within container openings of different sizes, and means to urge the locking member and protuberance inwardly, said protuberance having an upwardly inclined lower cam surface adapted to contact an adjacent portion of the container corner for restraining an upward movement of the container until a predetermined minimum lifting force is reached, said locking member being urged outwardly by contact of said lower cam surface with the adjacent portion of the container corner when the minimum lifting force is exceeded.