The Airslide was first patented in 1953, the same year Pullman Standard introduced their PS-2. What made the car unique was a set of fabric membranes in the hopper bays. Made of tightly woven cotton and treated with silicone, the Airslide® membranes were moisture-proof but allowed air to pass through. Compressed air was supplied at the unloading site and passed through the membrane up into the load. This aerated the load, allowing it to flow easily through the hoppers.

From the Lionel Trains official blog: Read more here