Why Steam Locomotives Had Small Wheels at the Front

Look at almost any classic steam locomotive and your eyes naturally go to the enormous driving wheels, the rods, and the cylinders, but the wheels that often decided whether that machine stayed on the rails were much smaller and tucked almost unnoticed beneath the front. They produced no power, hauled no train by themselves, and on many locomotives carried only a fraction of the total weight. Yet without them, fast steam engines could become unstable, freight locomotives could attack curves badly, and some early designs were notorious for simply leaving the track. Those little wheels were essentially the locomotive's guides, but the story of how they learned to guide hundreds of tons of machinery is far more interesting than it sounds.