ELECTRIFICATION The last decade of the 19th century saw the commencement of electric traction on railways and very considerable progress with railway electrification was made during the first twenty years of the 2oth century, both in Europe and in the United States. Electrification for railway purposes possesses many advantages, amongst which the most important are the elimination of smoke and steam, the greater power which can be obtained from any given axle load, quicker acceleration and the rapid reversal of trains as a result of being able to drive electric trains of the "multiple unit" design from either end, thus eliminating engine movements in a terminus where space and time are strictly lim ited. The abolition of steam and smoke is most desirable where trains are running wholly or mainly in tunnels or within the limits of large cities, and especially in stations closely encompassed either by office or residential buildings. Greater power is most desired where gradients are heavy, as in Switzerland and other countries where important main lines cross mountain ranges, while quick acceleration and elimination of engine movements in a terminus are very advantageous in connection both with the suburban traffic of main line railways and the underground lines in large cities. A further important advantage accrues to electric traction in countries which possess practically no coalfields of their own, but frequently own important water power resources which can be harnessed to produce electricity, and consequently render them independent of other countries for the traction power used on their railways. Fortunately railway lines with heavy gradients naturally exist in many areas where there are swift flowing rivers.
tion, each utilizing a different kind of current and possessing both advantages and disadvantages are (I) the continuous, or direct current system ; (2) the single phase alternating-current system; (3) the three phase alternating-current system. The direct-current system is that usually employed for suburban systems in connec tion with a comparatively low voltage, limited in early electrifi cation schemes to 600 volts, in which case a third rail was used from which the current could be collected by shoes. This system required the construction of many substations, but permitted light weight and small dimensions, and is consequently well suited to adoption with multiple unit trains where the axles of many of the cars on the train can be used as driving axles, thus obtaining great adhesion and permitting the train to be driven from either end. In many modern installations voltages of 1,200, 1,50o, 2,400 and 3,00o are now employed, in which case the current is collected from an overhead wire (while substations are designed to operate automatically, thus eliminating the need for attendants and can be shut down when not required) ; heavy currents can be collected from an overhead wire feed. Where traffic is dense a direct cur rent of 1,500 volts is advantageous and is found in the United States, France, Japan, Holland, Czecho-Slovakia and Great Brit ain. In cases where traffic is less dense but the trains very heavy, such as on the Chicago, Milwaukee, St. Paul and Pacific in the United States and in South Africa, Chile, Mexico and Brazil, 3,00o volts is employed. The Chicago, Milwaukee, St. Paul and Pacific employs regenerative braking which utilizes the force of gravity on the downward gradients to generate current for return to the power stations.