A freight carrier such as a railroad car is provided having sections which fold either upwardly or downwardly and inwardly from a stored position to an operational position which provides a supplementary level. Means are provided on the sections for supporting a subsequent section to be installed to aid a single operator in installing the supplementary floor. In the installed position, the sections have joining means which self-support the load and transmit lading shearing and bending moments between the sections so that additional supports from the floor are not required. In one embodiment the lowering into position of one section acts to disengage a subsequent section from a stored position so that it is ready for movement into the installed position. Several embodiments are disclosed directed to safety means to avoid the sections or platens releasing and harming the operator or others in the car.