FORMS OF TRAMWAY TRACTION Animal.—The original street tramways were worked solely by animal traction, the cars being drawn by one to four animals, according to the contour of the routes and traffic necessities. An average speed of six to seven miles per hour was customary; and the cost of horse maintenance and renewals averaged 4o% to 45% of the total working expenses.
Steam.—The need for higher speeds, quicker acceleration and larger and more comfortable cars than were practicable with ani mal traction, led to the evolution of a type of steam locomotive suitable for street tramway purposes. Some extraordinary designs were produced, not only for the engines themselves but for their position in the car. One inventor placed his engine at the front; another in the centre; and a third provided an engine at each end of the car. Between 1871 and 1883 many experiments on other forms of locomotives were carried out, including fireless loco motives in which well-insulated cylinders (filled with water which was heated from a stationary boiler at the end of each journey) together with the machinery were placed under the floor of the car. Ammoniacal gas and compressed air also found devotees but after many trials an enclosed steam engine coupled to the car became standardized, and rendered on the whole quite efficient service until displaced by electric traction. It was employed on most of the tramways in the manufacturing areas of England. Locomotive power and repairs and renewals of engines approxi mated 42% of the total working costs.
Gas.—The development of the gas engine for industrial pur poses seemed to offer possibilities for its introduction for street traction. Accordingly trials were made, principally in Germany, of small gas engines placed on the outside platform of the car, with the gas receivers accommodated under the floor, the gas being taken from the town mains and then put through a com pressing engine before being admitted into the cylinders.
Cable.—The principle of cable haulage was first applied to street tramways in 1873 in San Francisco by Andrew S. Holliday. It was introduced in England in 1884 on Highgate Hill, London, and installations followed subsequently at Brixton Hill and at Edinburgh, Birmingham, and Matlock. All these have since been discontinued, but there is still a small cable undertaking in opera tion at Douglas, Isle of Man. In cable systems haulage is effected by an endless wire rope continuously moving in one direction, supported on pulleys within a slotted conduit laid below the surface of the street or roadway, or between the rails of a surface or elevated railway. The rope is driven by means of a steam
engine or motor situated at a convenient place near the line, the motion of the cable being intermittently communicated to the cars for starting and stopping by means of a gripper attached to the car. The tube is formed of concrete with cast iron yokes spaced at intervals of 4 ft. to support the slot beams. The slot is usually in. wide and the conduit about• 19 in. deep x 9 in. wide.
Electric.—The history of the electric tramway is covered under ELECTRIC TRACTION. There also will be found a description of the engineering accomplishments that resulted in its superseding all other forms of tramways.
Much controversy used to occur as to whether the over head or conduit system should be installed. Engineers and finan ciers advocated the former on grounds of economy of cost of construction as well as of operation. But the general public was loth to sacrifice the amenities of the streets by permitting the erection of poles and overhead wires, and demanded a less ob trusive form of construction. In the end the economy and merits of the overhead system prevailed, the exceptions being confined to capital cities such as London, Washington, New York, Paris, Berlin, Brussels and a few other places where financial considera tions were subordinated to the aesthetic views. In this system the trolley wires are usually hung about 21 ft. above the rails. The poles are placed not more than 4o yds. apart.
Surface high cost of conduit construction and the objectionable character of the overhead wires respectively were incentives to the invention of an alternative method of con struction. Attention was given by engineering specialists to the surface contact systems and a number of designs were patented. Among those put into practical use in England were the Lorain system at Wolverhampton, the Dolter system at Torquay, Has tings, and Mexborough, and the G.B. system at Lincoln. On all these systems current is supplied from iron studs laid in the roadway between the rails of the track to a skate carried on the car. The studs were placed every io ft. to 15 ft., and contained a movable switch or contact which was operated by a magnet carried under the car.
Storage Batteries.—In theory storage batteries afford a simple and convenient form of electric traction, but extended trials have failed to justify their permanent adoption. Cars operated by batteries prove costly to work and maintain.