While a steam locomotive can simply start up from a stop by introducing steam into the cylinders, internal combustion engines must first be started with no load. Using the example of diesel engines, the engine must first be made to rotate sufficiently quickly by external force until it continues to run under its own power with ignition due to compression. This means that it is necessary to decouple the motor from the axles and only re-couple them before moving off. In addition, it must be possible to keep the engine rotational speed within an acceptable range over the entire usable speed range.

For smaller locomotives, there is a solution that is also used in road vehicles: a mechanical transmission, which usually has several gears and can separate the connection to the axles by means of a mostly mechanical friction clutch. The clutch can also be actuated automatically or be a fluid clutch. In this power range, a sufficiently gentle start-up can also be made possible.

Compared to hydraulic and electrical power transmission, mechanical power transmission primarily offers a cost advantage when purchasing. The power is then often transmitted via a jackshaft or directly to the driven axle and then to the other axles by means of coupling rods. Some locomotives also have a direct drive on several axles without coupling rods. 

A mechanical power transmission was mostly used in diesel locomotives in the power range up to around 200 hp, but there were also some examples with power around 400 hp. This basically divides the area of application of diesel-mechanical locomotives into small locomotives, which were mostly used in industrial plants or on field railways, and into mostly standard-gauge shunting locomotives, which had to move maller loads and had mechanical power transmission primarily for cost reasons. Technology from the construction of trucks and buses is also often used for diesel railcars, which also works with mechanical transmissions.