Learn English – Why do they call a murder a “red ball” case

In the nineteenth century, a common signalling arrangement was a ball hoisted on a mast or pole. These were for daylight communication with ships and later for railroads. Two common ball colors used were red and white. There were no industry-wide meanings for such signals; they depended on the rules of the port or road for definition. Typical uses were to show drawbridge status and to require waiting for a train from the opposite direction (as here).

Rail shipment delays were a problem for the railroads, particularly for perishable produce shipments from the west to eastern markets. In 1892, parts of the Santa Fe Railroad implemented priority treatment for certain designated trains. Whether this used, or just made reference to, a red ball track signal for the fast freight is unclear, but in any case the special trains were called red ball trains and came to have each car carry a placard with routing data and a large red ball (disc) printed on them.(source) By 1902, the Santa Fe had this system running on all its main lines, and by a decade later many railroads were using similar priority programs, often called "red ball" systems.

This railroad usage of red ball for fast transportation became widely known (since the railroads were important in that period). As trucking became established, some companies took on the term in their names: both a Red Ball Transit Company and a Red Ball Motor Freight were formed in the 1920s. Probably the most famous use of the term is the Red Ball Express, which was the truck routing system used in the Normandy campaign of World War II. This symbolic use of red ball for "priority handling" later came to be applied to important criminal cases.