Opinions as to the most convenient position and use of turn-tables have undergone a considerable modification during the past twenty or twenty-five years. Circular and semi-circular running-sheds for engines, as in [Figs. 436 and 437], are not so often adopted now as formerly. Although compact and accessible in theory, they possess the one great drawback that when the turn-table in the centre becomes deranged by wear or accident, none of the engines on the standing-lines inside the building can be taken out until the turn-table is again put into working order. A stock of from twenty to thirty engines might thus be put entirely out of the service for a day or more. This objection is considered to be of so serious a nature that running-sheds are now almost always constructed of rectangular form, of which [Fig. 438] is a type.
With this description of shed, the lines of rails are laid down parallel to one another, and the engine turn-table is placed on a line separate and distinct from those lines forming connections with the shed.
Where there is a large goods traffic, an endeavour is generally made to so lay down the goods-sheds and approach lines and sidings, that the full complement of waggons may be shunted in or out of the shed at one operation. This arrangement, which dispenses with turn-tables altogether, admits of the ready removal of a central or far-end waggon, without the necessity of taking out so many others in front one by one over the turn-table. At the same time, there are large numbers of these waggon turn-tables in use, and there are many cases where access to side sheds or detached stores can only be obtained by turn-tables.
A goods-shed and lines laid down with turn-tables, as in [Fig. 439], will always be more tedious and costly to work than one laid down with direct through lines, as in [Fig. 440]. Should either of the turn-tables shown on [Fig. 439] get out of order and become incapable of turning, then the entire side of the shed controlled by that table will be rendered useless until the defect be remedied.
Engine turn-tables are rarely made with more than one road on the top. The most modern types generally consist of two strong wrought-iron or steel-plate girders well braced together and securely attached to a middle framework which rests on and revolves round a centre-piece fixed on a solid foundation
To the ends of the girders are attached large roller wheels which travel round a solid iron or steel roller-path laid down along the circumference. These modern turn-tables are generally worked on the balancing principle, by bringing the engine and tender to a stand in such a position on the rails that the greater portion of the weight is thrown on to the cup-shaped steel centre, so that a small force applied to the long outrigged hand-levers at the ends is sufficient to turn one of the heaviest locomotives. [Figs. 441 and 442] give sketch plan and section of one of these steel-plate girder turn-tables, which has few parts, and very little to get out of order. The end rollers guide the table when making any portion of a revolution, and carry such part of the weight as may not be taken up by the centre. A recess is shown in side wall to facilitate the inspection of end rollers. In the earlier forms of engine turn-tables, the revolving movement was effected by attaching to the upper portion of the girders a strong winch, which acted upon gearing fixed either to the end rollers, or direct on to a toothed ring forming part of the roller-path. In cases where the engine turn-table was in constant use, as in connection with a large running-shed, the winch was sometimes driven by a small steam-engine to expedite the movement.
The great increase in the lengths and weights of modern locomotives has necessitated the removal of many of the old small turn-tables, and replacing them with others of 45 or 50 feet, or more, in diameter.
