Locomotives have their own braking system, independent from the rest of the train. Locomotive brakes employ large shoes which press against the driving wheel treads. With the advent of compressed air brakes, a separate system allowed the driver to control the brakes on all cars. A single-stage, steam-driven, air compressor was mounted on the side of the boiler. Long freight trains needed more air and a two-stage compressor with LP and HP cylinders, driven by cross-compound HP and LP steam cylinders, was introduced. It had three and a half times the capacity of the single stage. Most were made by Westinghouse. Two were fitted in front of the smokebox on big articulated locomotives. Westinghouse systems were used in the United States, Canada, Australia and New Zealand.