A torque tube system is an automobile driveshaft technology, used in cars with a front engine and rear drive. It is not as widespread as the Hotchkiss drive, but is still occasionally used to this day.
The "torque" that is referred to in the name is not that of the driveshaft, along the axis of the car, but that applied by the wheels, which must be transmitted to the frame by the suspension. The suspension of a Hotchkiss drive can transmit this torque; other suspension systems cannot use its driveshaft without additional suspension members. The cheapest system to do this (including the cost of suspension) is by means of a torque tube; the most expensive, a fully-independent rear suspension with trailing arms.
A torque tube is a hollow steel tube that extends from the transmission to the rear differential and axle. The rear end is bolted directly to the differential, where a Hotchkiss drive would have used a universal joint. The front is attached to the transmission with a "torque ball". The driveshaft, which transmits power and torque from the transmission to the differential, resides inside the torque tube. A single universal joint, located inside the torque ball, allows the driveshaft to flex slightly.
Drive thrust is transmitted through the torque tube to the transmission, engine, and thence to the frame. In contrast, a Hotchkiss system transmits this thrust to the frame at the rear differential. The fewer forces which must be transmitted in the vicinity of the differential by the torque tube made for a smoother ride (though modern independent suspensions are more advanced), which is an additional reason why it was used.
Examples of the torque tube were the American cars of the Ford brand up through 1948, which used the cheaper transverse springs that could not take the thrust. Buick started using coil springs in the 1930's, as did Nash's post-war Airflyte model; these also necessitated using a torque tube.