Behind the power and motion of every steam locomotive was a network of carefully engineered devices known as appliances. These components -- ranging from injectors and feedwater heaters to boosters, dynamos, and safety valves -- performed essential functions that made steam operation efficient, reliable, and safe. Though often overlooked beside the locomotive's driving wheels and cylinders, these systems represented the peak of mechanical ingenuity and remain a lasting testament to the sophistication of steam-era engineering.