As railroads around the world haul larger quantities of freight efficiently, the title of world's most powerful locomotive has often been passed to new generations of rolling stock.

There are many ways in which a locomotive can be the largest: the heaviest, longest, most cylinders, most power, or most wheels. It is often defined as the longest in length, but even then sources differ on whether the measurement should include the tender of a steam locomotive. A steam locomotive that has a tender cannot function without it, but (with rare exceptions) the tender does not contribute to traction. To establish the "largest" category, several factors take precedence: overall weight, which gives traction over driving axles; size (length and height of engine itself); and power, which may be in terms of raw horsepower, tractive effort, available power at axles (shaft horsepower) or, in the case of steam locomotives, available steam on a sustained basis.

• List of largest passenger vehicles