One of those idioms, like "dialing" a telephone, that has lingered long after the source has disappeared.
Decades ago each individual ride at Disneyland had a separate admission. In 1957, for example, the Dumbo Flying Elephant ride cost 25 cents. Disney sold ticket books that allowed people to save money. The tickets were labeled A through E. The very best rides, like the Matterhorn Bobsleds, were on that valuable E-ticket.
To this day, E-ticket means an exciting or sensational adventure. Even people who never held a precious E-ticket in their tiny hands and don't know the Disney connection use the phrase.
For people nostalgic for Southern California amusement parks, the blog Vintage Disneyland Tickets is a pleasant romp.