When crossing a desert camels are expected to carry their loads 25 m.
The substitution of steel for iron as the material for rails which made possible the axle loads and the speeds of Lto-day, and, by reducing the cost of maintenance, contributed enormously to the economic efficiency of railways, was one of the most important events in the history of railways, and a scarcely less important element of progressive economy has been the continued improvement of the steel rail in stiffness of section and in toughness and hardness of material.
It may happen that from a large station sufficient traffic may be consigned to certain other large stations to enable full train-loads to be made up daily, or several times a day, and despatched direct to their destinations.
It is used to a limited extent for mountain-grade goods traffic, and has the advantage over the " Consolidation " or eight-coupled type of lighter axle loads for a given tractive capacity.
On the other hand, where, as in America, the great volume of freight is raw material and crude food-stuffs, and the distances are great, a low charge per unit of transportation is more important than any consideration such as quickness of delivery; therefore full car-loads of freight are massed into enormous trains, which run unbroken for distances of perhaps 1000 m.