Shay locomotives were geared Steam locomotives that were designed and built by the Lima Locomotive Works in Lima, Ohio. They were a popular type of geared locomotive used primarily for logging and industrial applications during the late 19th and early 20th centuries. The Class D Shay locomotives were an improved version of the earlier Class C Shay locomotives, featuring several design enhancements.

Class D Shays had three cylinders and four trucks, and weight of 100 and 150 short tons. They were no more powerful than Class C, but had greater fuel and water capacity, resulting in improved adhesion.

The distinguishing feature of the Shay locomotives was their unique geared drivetrain, which allowed them to deliver high tractive effort and excellent pulling power on rough and steep terrain. This geared drive mechanism used a series of gears and drive shafts to transfer power from the steam engine to the wheels. The gears increased the torque available to the wheels, making them well-suited for hauling heavy loads and navigating challenging terrain.  These locomotives were particularly popular in the logging industry, where their ability to negotiate tight curves and steep gradients made them ideal for hauling heavy timber out of rugged and remote areas.  The Shay locomotives also found use in various other industries, such as mining, construction, and sugar plantations, where their reliability and power were valued.

Today, a number of Class D Shay locomotives have been preserved and can be found in museums, tourist railways, and private collections around the world. They stand as a testament to the ingenuity and versatility of these remarkable steam locomotives that played a significant role in the industrial and transportation history of the United States.