The cars of both styles of spreaders are loaded with old rails, frogs, scrap iron, etc., to hold them down and prevent derailments when hard or tenacious materials are suddenly encountered. Loads of five to ten tons are generally sufficient, though loads up to 15 tons are sometimes required.
Spreaders are usually drawn at a speed of six to eight miles per hour; in loose gravel the speed often reaches 10 miles per hour. They will level off a ridge a mile in length in six to ten minutes, doing as much work in that time as 100 men can do in a day.
The spreader is usually stationed in the nearest side track to the unloading place. Frequently it can be hauled between this track and the dump without raising it, or raising it only partially to clear depot platforms, switch stands and other obstructions and thereby avoid the necessity of folding it down on the car while passing between these points.
Ordinarily the spreading is done by the last train before the close of the day. In cold weather or on short dumps it must be done oftener; either to prevent freezing, or to make room for the unloading material which would otherwise pile up too high for easy spreading, or be liable to roll back on the track and obstruct it for the next train. In using the spreader it is coupled to the rear of the car carrying the plow, and after the train has been unloaded it is pulled over the length of the ridge of material unloaded from its own and preceding trains, as shown in Figs. 97 and 98.
The cost of steam shovel work varies greatly with the different conditions affecting each piece of work. It depends mainly upon the nature of the material, its location, the capacity and efficiency of the steam shovel, and the supply of empty cars or wagons. The efficiency of a steam shovel is not necessarily proportional to its capacity, but to the amount of work done compared to its cost; and while the amount of work done is generally larger in the machines of larger capacities, this advantage may be more than balanced by the greater cost of operation, including the cost of labor, fuel, supplies and repairs, etc. Machines of the largest capacity, with dipper of 2-1/2 cu. yds. capacity, are employed mostly in excavating soft materials, especially in loading gravel for ballasting. Machines of medium capacity are usually the most efficient for general construction work.