逆轉機
蒸汽機車上的逆轉機通常是一個操縱桿或舵輪，用於調整蒸汽在活塞的運作循環中，自汽室注入汽缸的持續時間。當逆轉機被完全向前、後扳轉時，蒸汽在一個活塞循環中幾乎不斷地注入。而當逆轉機扳轉於靠近中央位置時，蒸汽則只在循環中的一小部分時間注入。
With the regulator open, the vehicle’s chest is fed steam at a relatively constant rate from the boiler. However, the chest’s exhaustion rate to the cylinders increases with the vehicle’s speed, because the cylinder cycle becomes more frequent. At a certain speed, with the cutoff set to full forward, the chest no longer receives enough steam to overcome the exhaustion and the vehicle stops accelerating. However, by gradually pulling back on the cutoff the faster the vehicle goes, you give the chest increasingly more time to pressurize, making it possible to reach greater speeds.
Apart from setting the intervals in which steam is admitted to the cylinders, the cutoff control also determines the vehicle's direction, just like the reverser does on other powertrain types. Pulling the cutoff all the way backwards sends full steam in reverse. Cutoff must never be put in the opposite direction while in motion, as this sucks ash particulates and water into the cylinders, causing damage.