Air express is the oldest form of air freight but has now become a specialized category of air transport in its own right. There are records of short-lived or experimental services during the early days of the airlines, but regular or sustained air express service did not begin until 1927. Subsequently, the service grew rapidly in popularity, mainly after the different companies had been consolidated into the Railway Express Agency.This Agency always conducted its air express service through the cooperation of the certificated air lines and prospered until the late 1960s. It was superseded by the emergence of a new class of airline which specialized in overnight package service, using its own fleets of aircraft. Today, six leading competitors in the business are operating more than 300 aircraft which concentrate exclusively on the carriage of air express.