. Railroad construction. Theory and practice . ofthe lead which is used for the couater-balance weight. Incidentally severalother features and dimensions are shownin the illustration. 326. Mutual relations of the boiler power, tractive power,and cylinder power for various types. The design of a locomo-tive includes tliree distinct features which are varied in theirmutual relations according to the work which the engine isexpected to do. (a) The boiler power. This is limited by the rate at whichsteam may be generated in a boiler of admissible size and which are designed to haul v
. Railroad construction. Theory and practice . ofthe lead which is used for the couater-balance weight. Incidentally severalother features and dimensions are shownin the illustration. 326. Mutual relations of the boiler power, tractive power,and cylinder power for various types. The design of a locomo-tive includes tliree distinct features which are varied in theirmutual relations according to the work which the engine isexpected to do. (a) The boiler power. This is limited by the rate at whichsteam may be generated in a boiler of admissible size and which are designed to haul very fast trains which arecomparatively light must be equipped ^^ ith very large grates andheating surfaces so that steam may be developed with greatrapidity in order to keep up with the very rapid for Aery heavy freight work are run at very muchlower velocity and at a lower piston speed in spite of the factthat more strokes are required to cover a given distance andthe demand on the boiler for rapid steam production is not. Fig. 199.—Section ofLocomotive-DRIVER. 362 RAILROAD CONSTRUCTION. § 326 as great as with high-speed passenger-engines. The capacity ofa boiler to produce steam is therefore Hmited by the hmitingweight of the general type of engine required. Although im-provements may be and have been made in the design of fire-boxes so as to increase the steam-producing capacity withoutadding proportionate!}^ to the weight, yet there is a more orless definite limit to the boiler power of an engine of givenweight. (b) The tractive power. This is a function of the weight onthe drivers. The absolute limit of tractive adhesion between asteel-tired wheel and a steel rail is about one third of the pressure,but not more than one fourth of the weight on the drivers canbe depended on for adhesion and wet rails will often reducethis to one fifth and even less. The tractive power is thereforeabsolutely limited by the practicable weight of the engine. Insome desig
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