At the beginning of the railway era, the locomotives were used interchangeably for any kind of task, either towing heavy loads of minerals or coal or otherwise passenger cars. However, very soon the railway companies understood that, to increase the performance of those machines, new types should be built, adapted to the different functions. It was demonstrated that, either for the transport of voluminous and heavy freight cars or for the travels accross the rough reliefs of some mountainous regions, where powerful machines capable of climbing the steep slopes were needed, the ideal solution would be to increase the number of driving wheels. Thanks to this method, the huge weight of a gigantic locomotive could be distributed on many axes, allowing it to transit on relatively weak railroads; besides, the supplementary friction produced by the contact of the wheels with the rails would increase the traction power.
Canadian Pacific 2-10-4 locomotive, 1949.
For example, the gigantic Canadian Pacific 2-10-4, which operated with heavy oil and had cylinders of more than 60 centimeters in diameter, with a weight - tender included - of about 330 tonnes, was capable of dragging convoys of more than 1500 tonnes accross the roughest areas of the Rockies, albeit, sometimes, this very machine needed the assistance of an auxiliary machine to reach a summit at 3000 meters of altitude. Starting from the reasoning that a wheel of larger diameter covers a larger distance, in each revolution, than a wheel of lesser diameter, the British engineer Crampton patented in the 1840s a locomotive which had, in its rear part, only a pair of driving wheels... of almost 2.5 meters in diameter each! The locomotives designed by Crampton were used in Europe during long time, albeit the number of driving wheels was progressively increased, from two to four and finally to six, to achieve better adherence and stability.
The super-fast 4-4-4 locomotive of the line Baltimore & Ohio (whose chimney topped in brass was inspired in British models) was built in 1934 with wheels of more than two meters in diameter. Fitting the locomotives with so large wheels was the easier way to achieve speed, so this method was common already in the 1850s.
In the short routes or in the shunting yards, the tenders resulted more obtrusive than useful, so the machines destined to those services transported the coal and the water in storages placed at each side of the boiler. These locomotives were usually small and, since the frequent stops and start ups required a perfect adherence, all the weight of the compound "machine-tender" rested on numerous driving wheels of small diameter. On the other hand, to travel accross wild or deserted regions, where fuel supply centers were located distant to each other and where it was habitual the existence of unstable track sections or tight turns, were required locomotives of great resistance and perfect stability.
Tank locomotives (also known as tender locomotives, albeit this denomination is regarded as incorrect by experts) were developed for being used in commuter lines or switching yards, where their compact design with both coal and water storages fixed to the machine allowed for easier maneuvering. These machines started to be common in British railways in the 1870s.
These very characteristic models of locomotives with Garratt articulation were used throughout Africa from the 1920s to the last days of the steam railway. This design allowed a machine fitted with a large fuel storage to be able to operate in lines with relatively tight turns, where machines fitted with a large one-piece tender and simple articulation could not. Even if these special machines were a British speciality, the ones of better performance were built in France for serving in the rough Algerian deserts.
The response to the search of a further refinement in the railway transport was the articulated locomotive. The system, devised by Anatole Mallet, consisted of a main chassis with a truck of driving wheels and a secondary chassis, fitted as well with a truck of driving wheels, articulated over the first one. These fixed driving wheels were completed with bogies and support axes. By means of this disposition, were obtained a strong adherence on the steep inclinations of the terrain and great flexibility for the accommodation of the powerful locomotives in the curves of reduced radius. Mallet locomotives were designed in France, circulating firstly in the peripheral boulevard of Paris and later in the railway networks of other nations, mainly United States, where it was adopted the model 2-10-10-2, of 250 tonnes. On the other hand, while studying a system destined to mounting heavy artillery pieces on bogies, Herbert Garratt designed a new model of articulated locomotive, whose operation was based mainly in two driving units which obtained the steam from a common boiler placed between the two.
The largest locomotives that circulated in regular service were the Big Boys 4-8-8-4 of the Union Pacific, introduced in 1941. These machines, fitted with Mallet articulation and simple expansion, weighed 350 tonnes, developed 6000 horsepower and consumed 22 tonnes of coal and 44 tonnes of water each hour.
A century lies between the small Fussée (less than 6.50 meters in length) and the gigantic Big Boy (more than 40.50 meters in length and 350 tonnes of weight). Between them appears the Chapelon 4-8-0 of the Paris-Orléans Company, introduced in 1940; despite its modest dimensions (20 meters in lenght and 110 tonnes of weight) this locomotive developed a power equivalent to more than two thirds of the one produced by the Big Boy, achieving a never equalled performance for a steam engine, regardless of further technical progress.