Why the path to expansion engines was not linear

Steeple, Siamese, direct-acting, oscillating, and other engine types reveal the variety hidden by modern hindsight. The expansion engines most students picture represent machinery refined after steam engines had already been in service for nearly a century. Development did not proceed neatly from single to double and triple expansion before arriving at turbines; engineers explored many branches before particular arrangements became dominant. Even proposed quadruple or quintuple expansion systems depended on boiler pressures that could instead support a high-powered turbine, although their aim was to extract as much energy as possible from the steam. Cultural memory preserves mature technology and compresses the difficult, experimental routes that produced it.