The Many Nicknames for Chicago's Elevated Railway
Edward McClelland surveys the various references to Chicago’s elevated rail system over the years. Authors writing about Chicago—including James T. Farrell, Nelson Algren, and Gwendolyn Brooks—have called it a variety of names: the el, the El, and the L.
But the Chicago Transit Authority is clear about what it considers the right name. "The term 'el' can be short for 'elevated railway' generically, but our system has used 'L' since the 1890s. This proper, official nickname extends to elevated, at-grade, and underground tracks, and is used on official CTA materials."
McClelland notes that using L can confuse readers and El probably makes more sense. "In New York City, which assigns letters to its subway routes, the L train runs from Eighth Avenue in Manhattan to Rockaway Park in Brooklyn. (Also, in Chicago, the letter L has the negative connotation of appearing on blue flags after Cubs losses.)"