. After the 1967 SCL merger, these cars were fitted with
rotary couplers and used in
Bone Valley phosphate service. The word "hopper", meaning a "container with a narrow opening at bottom", goes back to the thirteenth century, and is found in
Chaucer's story "
The Reeve's Tale" (written late fourteenth century) in reference to a machine for grinding grain into
flour. Historically, open hopper cars were used to carry coarse mined products like
coal,
ore, and
gravel, while
boxcars were used for granular materials requiring protection from the elements. Weatherproof covers were added to hopper cars, creating the covered hopper. Early production emphasized two-bay cars very similar to open coal hoppers and suitable for materials of similar density, like
Portland cement or
rock-salt. Some cars were available in the 1910s, and became more common by the 1940s. These early cars were volume-limited for less dense commodities like grain or
sugar, so later designs include longer covered hopper cars with higher sides and three or more bottom bays. Increasing
axle load limits have allowed some of the heavier loads formerly assigned to two-bay hoppers to be assigned to larger, more efficient three-bay hoppers. Some covered hopper cars retain the conventional centersill as a strength member transmitting
compression and
tension forces from one car to the next. Beginning in the 1960s, designs distributing these forces along the sides of the car eliminated the centersill beam to simplify
bulk material handling with wider hopper openings reducing the tendency for bridging to restrict gravity flow when unloading the car. == Typical North American freight car weights and wheel loads ==