RACK LOCOMOTIVES are used on roads where the grades are too steep to be operated by the ordinary traction locomotive. The adhesion be tween the wheel and the rail of such an engine cannot he taken at more than 25 per cent. of the weight on the drivers. Hence when the grade be comes so steep that the resultant of gravity act ing parallel to the wheels becomes equal to 25 per cent. of the weight on the driving wheels it will be impossible for the locomotive to advance. Such a grade is reached when the rise is1320 feet to the mile. Practically it is much less than this. So on a grade of onedtalf of this the engine could at hest only hold. not hanl, itself and a load equal to its own weight. Hence for roads climbing mountains where the gradient is neces sarily very steep the rack locomotive is used. Such an engine is shown in side elevation in Fig. 17. The weight of the locomotive is car ried on smooth rails in the ordinary manner, and while the grade lies within the limits of its weight to overcome, it is propelled by driving wheels like other engines. But when the grade becomes too steep, the gear-wheel is brought into mesh with a rack rail laid in the centre of the track between the other two. The teeth of this gear engage with those of the rack and propel the engine independently of the over which developed lit the driving wheels and which would be in sufficient to do the work. 'Between the two wain driving axles is another driving axle, carrying a gear-wheel, which meshes with the rack.
This gear-axle is driven through reducing, ing by a pair of inside cylinders. In addition
to the two main axles there is another rear axle carrying, besides its pair of wheels, a brake pinion. The crank-pins are made spherical so as to afford the necessary flexibility when rounding sharp curves. All four cylinders have the same diameter and length of stroke and are inde pendent of one another so far as steam-supply is concerned. When running along the ordinary track the inner cylinders have no steam-supply and do not perform any work.
In the matter of brakes there are the ordinary brake-shoes on the driving wheels and strap brakes on the rear locomotive truck axles, and air brakes on the cars. Such an engine as that shown in Fig. 17 is calculated to haul 40 tons up a 90 per cent. grade at a speed of five miles an hour. This class of engine is extensively used on the mountain railroads of Switzerland. It is also used on the roads running up Mount Wash ington and Pike's Peak in the United States.
BIBLIOGRAPHY. The early history of locomoBibliography. The early history of locomo- tive construction and performance is given very fully in Colburn, Locomotive Engineering (Lon don, 1871). Interesting chapters on locomotives are found in Thurston, Growth of the Steam En gine (New York, 1878; 4th ed. 1897). Meyer. Modern Locomotive Construction (New York, 1892). and Whitridge. Modern Locomotives (New York, 1897), give a large amount of infor mation regarding modern practice in locomotive construction. A small book of unusual value is Reagan's Locomotive Mcchanis»z and Engineer ing (New York. 1903).