In 1915 approximately 1,044 miles were add ed to the electric-railway mileage of the United States, as compared with 946 in 1914. Of the total added in 1915, 596 miles repre sented new electric-railway track built, and 448 miles electrified steam mileage. According to the Electric Railway Journal the total electric railway mileage of the United States in 1916 was approximately 46,454 miles; and the elec tric-railway companies operated 99.405 cars and locomotives. There were 1,027 companies operating the total mileage of 46,454, with 98,893 cars and 512 electric locomotives. Com plete data of electric-railway earnings are not currently available, the latest detailed and com plete statistics being those of 1912 as furnished above.
Many problems in electricity have arisen in the development of the street railway, the latest of which is the question of the alternat ing current. It is readily seen that an alter nating current system of electric traction which would permit the use of a high transmission line potential and avoid the transformation to direct current by means of the rotary converter would give an impetus to the electrical solution of transportation problems greater even than that which followed the introduction of the rotary converter. The direct current, because of its limitation as to voltage, is inadequate as a system for the distributing of power over a wide area. The alternating current system, permitting a change of potential by means of the simple and efficient static transformer, has already successfully met the requirements of transmission and distribution of electric power. The necessity of using direct current for the operation of cars has, however, greatly re stricted its application to general railway serv ice, involving, as • it does, the introduction of the rotary converter with its auxiliary appa ratus, increasing the cost both of installation and operation and reducing the commercial efficiency of the system as a whole. However, the direct current was in use in 1916 on nearly all city lines and on a large proportion of the interurban lines. The city lines are operated at 500 or 600 volts and the suburban lines at from 1,200 to 1,500 volts. It is claimed for the alternating current system that it saves 20 per cent in power required. The cars, how ever, are more expensive and this tends to off set the cheaper equipment required in alter nating sub-stations.
Catenary line construction is intended for high tension trolley roads, operated at poten tials up to 6,000 volts or more. It is especially
designed for use in conjunction with single phase alternating current railway equipment and marks one step further in the movement to secure a greater degree of economy and efficiency in the operation of electric trolley lines. The employment of high tension cur rents for traction purposes necessitates the use of an improved trolley equipment possessing an efficiency and a reliability of a high order. The speed attained upon interurban lines makes it difficult to obtain satisfactory service with a trolley wire which dips between sup ports and sags and sways with every impulse. Increased precautions against accidents and faulty construction are also necessary because of the increased liability of damage from any diversion of the line current from its proper channel. Study of the traffic reports of many roads has shown that the passenger traffic to be expected on street railways in the United States ranges from 70 rides (fares) per capita per year in cities of 15,000 population, to 240 fares per capita in cities of 1,000,000 popula tion. The intermediate figures are: 120 fares per capita in cities of 50,000 population; 150 fares to each of 100,000 population; 180 fares to each of 200,000 population; 210 fares to each of 300,000 population; 220 to each of 500,000 population.
In late years there has been a rapidly in creasing demand for a controlling system ap plicable to trains of motor cars as well as to cars operated singly. This demand has been met by the development of the unit switch system of multiple control, which consists of a skillful combination of electromagnetic and pneumatic devices, each applied to those oper ations in which experience has shown it to be most effective. The construction is noticeable for the liberal design of its working parts and contacts and the great margin of power avail able for their operation; while the general de sign and simplicity of operation insures great reliability of service and low cost of mainte nance. See STREET RAILWAYS.
For extended discussion of technical and operating problems, consult Bureau of the Cen sus Bulletin 124; Austin's 'Single Phase Elec tric Railways' (1915) • Harding's 'Electric Railway Engineering' (1916) ; Norris's