Opinions as to the most convenient position and use of turn-tables 289 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.