It became an iconic symbol of Grateful Dead because the show opening lyrics to Casey Jones are “Drivin’ that train, Casey Jones”. The protagonist’s name is derived from a folktale about a man who drives his locomotive recklessly, and consequently wrecks it brutally killing both men.
The tale has been lurking in the shadows of popular culture for centuries. There was even a myth around US railroad workers attributed to “Casey Jones”, coming from the ‘mid 19th century source “Mrs Porter’s Neckties”‘.