The earliest express-service refrigerator cars entered service around 1890, shortly after the first express train routes were established in North America. The cars did not come into general use until the early 20th century. Most units designed for express service are larger than their standard counterparts, and are typically constructed more along the lines of baggage cars than freight equipment. Cars must be equipped with speed-rated trucks and brakes, and — if they are to be run ahead of the passenger car, must also incorporate an air line for pneumatic braking, a communication signal air line, and a steam line for train heating. Express units were typically painted in passenger car colors, such as Pullman green.