The 5660 Pressure Differential (PD) Covered Hopper was introduced in 1999, by Thrall Industries, as the most advanced design in the evolution of Pressure Differential Covered Hopper, a special breed within the covered hopper family. Production still continues today under the TrinityRail brand, following Trinity’s purchase of Thrall, with nearly 4,000 examples already built.
These PD covered hoppers, unlike traditional “gravity discharge” covered hoppers, feature five non-opening hopper bays all connected by a master commodity discharge pipe and an air inlet pipe spanning the length of the underbody. Unloading of these cars is an orchestrated process: an air source is connected to air pipe, which pressurizes the interior of the car to approximately 15 psi, although 14.7 psi is recommended for the Thrall/TrinityRail 5660. As air pressure inside the car’s body begins to build and exceeds that of the external environment, the basic physics of differences in air pressure (hence the name “Pressure Differential Covered Hopper”) forces the air outside of the car, whisking the car’s contents along with it through the commodity discharge pipe. This type of unloading is necessary for the seamless unloading of dry, powdery, light-density commodities such as flour, starch, talc, fly ash, bentonite, and other powdery substances which would otherwise “cake up” if being unloaded from a traditional gravity-discharge covered hopper.
Given the commodities these cars were designed to transport, the Trinity 5660 Pressure Differential Covered Hopper is a common sight at flour mills, bakeries, cereal plants, food processing plants, bulk transfer terminals, and even simple team tracks. At large facilities, the car’s interior is pressurized and the contents are directed into storage silos through an industrial-grade hose connecting the outlet on the commodity discharge pipe with the silos. At bulk transfer terminals or team tracks, a pressure differential (PD) semi-truck – usually provided by the end customer – pulls up to the side of the car, injects air into the car using the truck’s generator, and transfers the commodity through a hose connecting the railcar and the semi-trailer. From loading to unloading, the commodity is always protected from contamination, ensuring the purity of the food ingredients is not compromised. In fact, most 5660 Covered Hoppers in service require a thorough cleaning of their interiors upon returning to their shippers and taking on another load. Many flour mills have a dedicated “cleanout track” simply to sterilize the car’s interior prior being assigned another shipment.
Aesthetically, the 5660 is a work of art: the car’s mid-section is smooth and elegant, and is immediately contrasted by sharp and pronounced angled braces towards its end cages. This five-bay PD car boasts an undeniably massive and hulky appearance featuring a more tenacious look than many other covered hopper designs. The sheer size of the car is impressive: 63’, 5” over the strikers, 15’, 6” in height, 10’, 4¾” wide with prominent cylindrical side sheets at a 40 degree slope, and an average light weight of 67,500 pounds. Despite its size, the car still manages an AAR Plate C designation. The 5660 also enjoys the distinction of being the largest, highest-capacity, and most modern iteration of the Pressure Differential breed of covered hopper traversing the rails today. This gives the 5660 a distinct advantage over its predecessor cousins, all earlier versions of lower-capacity PD cars produced by Thrall, Trinity, NSC, NACC, and ACF’s popular “PressureAide”. Additionally, the 5660 is forcing the retirement of many aging “Airslide” covered hoppers which were designed to haul the same commodities.
Make no mistake, the Trinity 5660 Pressure Differential Covered Hopper has become the new standard for shipping powder-type substances, and has come to define the essence of modern railroading as we know it. Simply stated, it is the most advanced model of Pressure Differential covered hopper technology today