The story
From red + cap. In sense of "porter", 1890s US; compare contemporary Japanese 赤帽 (akabō), Chinese 小紅帽 /小红帽 (Xiǎo Hóng Mào). On Labor Day, 1890, John Williams, an African-American railway porter, tied a red ribbon to his black uniform cap to stand out from the crowd at Grand Central Terminal. The strategy was so successful that it was soon adopted by others in the profession, leading to the synecdochic use of redcap as a term for all railway porters.
Etymology from Wiktionary, CC BY-SA 4.0.