built into the passenger cars. Reversal of such trains may be cut down to about three minutes, where the trains are very short, or where a second driver is ready to enter the rear of the train as it arrives and drive it on the return journey. More rapid accel eration, together with the greater ease of fitting automatic stop signal devices, permits a closer headway than with steam trains, where frequent stops are concerned, whilst for tunnel sections and underground railways electric traction has come to be regarded as essential. The question as to whether to equip a steam railway for electrification is an ever present problem to many railway ad ministrations and each individual case must be studied on its own merits. Against the advantages of electrification are high capital cost of equipping the line and adapting the rolling stock, while from the operating point of view it is a great advantage to possess each motive unit independent (as in the case of the steam loco motive) of any central power supply. The best method of com paring the two sorts of traction is by means of the net revenue, that is to say, the profit on operation. Electrification invariably attracts traffic, especially passengers, hut it is usually more costly to operate, owing to the high interest charges on capital ; hence its success will depend upon the amount of new revenue it can attract.
Underground and Elevated Electric City Railways.—City railways may be distinguished from tram or street railways in that they possess their own right of way, either above the streets in the form of elevated lines, or below ground in ordinary tunnels or, as in the case of many London railways, in tubes. Such rail ways normally only carry passengers. Their average journey is short, and the capital cost of construction per mile very high. The traffic is very unevenly spread throughout the day, and the speed and frequency of trains must be sufficiently great to enable them to compete with road services offered by omnibuses, tram ways, or even walking. In Great Britain the only important
elevated railway section is that of the Liverpool Overhead, claimed as the first electric overhead railway in the world and opened in 1893, 15 years after the elevated system appeared in New York. Sections of elevated lines also exist in other large cities, such as Paris, Berlin and Boston. Electric underground passenger rail ways exist in Great Britain almost wholly in the London area, an important exception being the Mersey Railway which, originally steam, adopted electric traction in 1903. The first London tunnel railway was the Metropolitan running between Bishop's Road and Farringdon Street and opened in 1863, from which the Metro politan and the Metropolitan District lines of to-day, largely constructed on the "cut and cover" principle, developed. The "Inner Circle" section of these lines and the extension to Ux bridge was electrified in 19o5, and since greatly extended. The City and South London, opened in 189o, was the first deep level line to be built in Great Britain on the "tube" principle, by means of shields invented by J. H. Greathead. Electric locomotives were used on the line, but like the later tubes, all trains are operated with multiple unit trains. In the case of the tube lines, amounting to about so route miles, each track runs in a single tube, with the stations built in tubes of large diameter. On the Central London gravity is used to assist the slowing and acceleration of the trains on arrival and departure at stations. New York, Paris, Berlin and Budapest all have underground electric railways, but London's system is primarily of the deep level type. Glasgow also has an underground line, while Chicago has a network of underground freight lines. London possesses a unique Post Office miniature underground electric railway, the system being worked from the stations with no crews on the trains. The cost of construction of such lines depends entirely upon geological strata and the extent to which sewers and underground streams are encountered, but in London it has varied between £250,000 and LI,000,000 a mile. The system of operation is quite distinctive from that of main line or even outer suburban railways, for speed and weight of trains are uniform at any one time, few trains pass stations with out stopping, and the "headway" between trains is very close.