The brakes themselves were actuated by the pressure of compressed air held in a reservoir, whereas the steam in the title referred to their use on steam-powered locomotives.
His original patent for his invention of the air brake was issued 13 Apr 1869 (No.88,929) for the use of compressed air fed from a reservoir tank along pipes to brake-cylinders and pistons, attached to each car.
His improvement with patent 124,405 providing each car with an auxiliary air reservoir filled from the main reservoir and able to be used independently for each car. Air (vacuum) brakes have have since been further developed for big trucks, buses and amusement park rides, among other applications.
According to todayinscience