OVERHEAD-TROLLEY RAILWAYS. The vast ma jority of electric railways in operation at present are overhead-trolley lines; that is. d• current for operating the cars is taken from an overhead wire •ondueto• by means of an under-running trolley-wheel carried on a trolley-pole extending upward from the car roof. The current passe; down the t•olley-pole conductor to and through the motor, and returns to the central station by way of the rails. Tn a few instances double trol ley lines have been constructed which the second conductor provides for the return eur :ent. Trolley-road comdruetion may lie sob divided into roadbed and track construction. and overhead construction. Only a few salient fea tures of each can be touched upon here. Strect• railway tracks when onoe laid cannot be re paired without taking up the pavement : and as this is all expensive operation, great care is taken in first putting down the tracks. Late practice for city work is to use standard oak ties 6Xs inches by S feet long. spaced about IS for (-very 30 feet of track. A trench is usually excavated for each tie to a depth of from four inches to six incites below the bottom of the tie. The bottom of this trench is tilletl with broken stone and the tie laid upon this filling. The rails are then laid and spiked to the ties, a girder rail nine inches deep being commonly employed. These rails are usually either 30 or 60 feet long. and are con nected structurally either by splice plates or cast joints, or it electric welding, and, electric ally, where splice joints are used. by means of various forms of rail-bonds. Splice joints for electric railways resemble those of steam rail ways, and consist of two splice plates for each joint, which are placed on opposite sides of the two abutting rails and clamped together by bolts passing through both splices and rails. The method of making cast welded joints is as fol lows: The rails at the joint are cleaned and brightened by a sand-blast (q.v.). and a cast
iron mold is placed around the rail-ends. making a tight fit. Into the mold the molten metal is poured. The metal in contact with the mold begins to cool and forms a crust, while the in terior remains in a molten state. This crust con tinues to cool and at the same time contracts. forcing the molten metal toward the centre of the joint. The top or hearing surface is after wards filed off perfectly smooth, so that it is difficult to detect a joint by riding over it. Elec tric welding of rail-joints is described in the article on WELDING. The rails being laid and the jointing and bonding finished, the space be tween the ties is ballasted and the pavement laid, restoring the street to its original condi tion. For suburban and interurban roads. a track built of from 60-pound to TO-pound spiked directly to the ties is used. the road sur face being restored after all is completed.
Overhead construction for trolley roads com prises the electric conductor and the structures for carrying the conductor. The supporting structure for the conductor is of two forms, usually called span-wire and bracket-arm. In the first the trolley-wire is hung from a span wire stretched between poles on opposite sides of the track; in the second, a single row of poles is used with a bracket-arm extending out over the track to carry the conductor. Often double-bracket poles placed between the tracks carry the two conductor, for double-track roads. Both wood and iron poles are used. In America the development of overhead eon-truetion has been in the direction of securing lightness. strength. and eir,tivene-- at low cost. and ornamentation has not been sought. In Great Britain and Continental Europe, however. fancy brackets, scrolls, and ornamental castings have been used very generally, and this is the essen tial difference between the practice on the two sides of the ocean. Car and cent ral station con struction are described in succeeding paragraphs.