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Northern Pacific Railroad Shops

Inside a Diesel shed

Motive power depot, usually abbreviated to MPD, is a name given to places where locomotives are stored when not being used, and also repaired and maintained. They were originally known as "running sheds", "engine sheds", or, for short, just sheds. Facilities are provided for refuelling and replenishing water, lubricating oil and grease and, for steam engines, disposal of the ash. There are often workshops for day to day repairs and maintenance, although locomotive building and major overhauls are usually carried out in the locomotive works.

MPDs in Britain are now often known as traction maintenance depots.

German practice[link]

The equivalent of such depots in German-speaking countries is the Bahnbetriebswerk or Bw which has similar functions, with major repairs and overhauls being carried out at Ausbesserungswerke. The number of these reduced drastically on the changeover from steam to diesel and electric traction and most modern Bw in Germany are specialised depots, often responsible for a single rail class.

See also[link]

• Ausbesserungswerk
• Bahnbetriebswerk
• List of British Railways shed codes
• Roundhouse
• Traction maintenance depot

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A young "drawer" pulling a coal tub up a mine shaft, whose "effect" constitutes the "elevation of a weight to a certain height" (Sadi Carnot).

In thermodynamics, motive power is an agency, such as water or steam, used to impart motion. Generally, motive power is defined as a natural agent, as water, steam, wind, electricity, etc., used to impart motion to machinery; a motor; a mover. The term may also define something, as a locomotive or a motor, which provides motive power to a system. In current use, motive power may be thought of as a synonym for either "work", i.e. force times distance, or "power", an effect producing motion, depending on the context of the discussion.

History[link]

In 1679, physicist Denis Papin conceived the idea of using steam to power a piston and cylinder engine, by watching a steam release valve of a bone-digester rhythmically move up and down. In 1698, based on Papin’s designs, mechanical designer Thomas Savery built the first engine. The first scientific treatise on the energetics of engines was the 1824 paper: Reflections on the Motive Power of Fire written by French physicist Sadi Carnot.

As an example, the Newcomen engine of 1711 was able to replace a team of 500 horses that had “powered” a wheel to pump water out of a mine, i.e. to “move” buckets of water vertically out of a mine. Hence, we have the precursory model to the term motive power. Based on this model, in 1832, Carnot defined work as “weight lifted through a height”, being the very same definition used to this day.

1824 definition[link]

Carnot states, in the footnotes to his famous 1824 publication, “We use here the expression motive power to express the useful effect that a motor is capable of producing. This effect can always be likened to the elevation of a weight to a certain height. It has, as we know, as a measure, the product of the weight multiplied by the height to which it is raised.”

In this manner, Carnot is actually referring to "motive power" in the same manner we currently define "work". If we were to include a unit of time in Carnot's definition, we would then have the modern-day definition for power:

Failed to parse (Missing texvc executable; please see math/README to configure.): P=\frac{W}{t} = \frac{(mg)h}{t}\

Thus Carnot's definition of motive power is not consistent with the modern physics definition of "power", nor the modern usage of the term.

1834 definition[link]

In 1834, the French mining engineer Emile Clapeyron refers to Carnot’s motive power as “mechanical action”. As an example, during the expansion stroke of a piston engine he states that: “the gas will have developed a quantity of mechanical action during its expansion given by the integral of the product of the pressure times the differential of the volume.” Clapeyron then goes on to use graphical methods to show how this "mechanical action", i.e. work in modern terms, could be calculated.

See also[link]

• Power (physics)
• Thermodynamics