In the wide world of covered hoppers there are many variations and patented innovations which make cars better suited for specific commodities. These also make train watching a lot more interesting for all of us! General American’s “Airslide®” cars are certainly near the top of the list of these important designs.
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.
An article in Railway Age in 1956 touted the cars’ loading speed,
“In actual test, one major company found that one man unloaded 101,100 lb of material from an “Airslide” car in 2 hours, 18 minutes. A standard hopper car, meanwhile, with 70,000 lb load and two men working, required 5 hours, 15 minutes to unload.
“General American reports that, at least up to this time, it hasn’t found a conveying system with capacity to unload the car at its maximum potential speed.”
The conventional covered hopper was not the Airslide’s only competition. Many shippers were still using bagged loads in boxcars for these types of loads. Many shippers found a use for the cars. Flour, sugar, and other soft, dry granulated materials are common loads.
Two types of hoppers were available. Some had a simple gravity outlet which dumped the contents straight down. They could be unloaded into a pit between the rails or onto a conveyor or auger placed under the car. A pneumatic unloading port was also available on the hopper. A hose could be attached to this for pneumatic unloading directly from the car.
By using these devices, the Airslide didn’t require as steep a slope in the hoppers themselves as in conventional covered hoppers of the day. This allowed more space for increased capacity without increasing the length of the car. Placing all of the structural components, except the center sill, on the exterior sides and roof also made the interior easy to clean.
General American built its first demonstrator in 1953 and production began in 1954. Many of the first cars were built for their own lease fleet. Many of these in turn were put on long-term lease to private shippers and railroads. These cars would have shippers / railroad logos and graphics but GACX reporting marks and numbers. Many of the railroads that initially leased the cars ended up buying them outright.
Both 2600 and 5300 cu ft. cars were offered. The 2600 cu ft (also commonly called the “single-bay” or “40 ft” Airslide) was the most popular for the first decade. Approximately 5000 cars of this size were built up until 1969. The cars featured a single hopper on each side of the center sill. From the side sill up, the cars had a more boxcar-like look than other covered hoppers of the day. The ends featured open areas inside the safety rails and ladders which provided access to the loading hatches on the roof. Air brake equipment was located in this space on one end of the car.
In the 1960s and 70s, more shippers began opting for the 5300 cu.ft. version of the car. These featured a pair of discharge outlets on each side and were 50′ long.
As the demand for the larger cars made the earlier 40′ cars surplus, many of these were semi-permanently coupled with drawbars into paired single-bay cars. Some of the most recent conversions have been even more simple – only the coupler cut levers have been removed to make the semi-permanent connection.
Between the paired 40′ and more common 50′ cars, there are still hundreds of Airslides on the rails today. Some are owned by railroads, many are privately owned or are part of GATX’s own lease fleet.