It goes on to say, "To provide greater security, the Department began using wire-caged mail wagons in the late 1880s. While the mail was being moved, a lock secured the back doors of each mail wagon. Because of this design, the wagons were quickly nicknamed 'screen wagons'. Postmasters found the screen wagons easier to maintain and cheaper to operate. Another benefit to postmasters was the wagon’s tight turning radius. The front wheels could turn completely under the body, allowing the vehicles to easily pivot into tight loading docks."
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To watch a short film of screen wagons being loaded with mail at a post office in 1903, click here.