16. No station to be constructed, and no siding to join a passenger line, on a steeper gradient than 1 in 260, except where it is unavoidable. When the line is double, and the gradient at a station or siding-junction is necessarily steeper than 1 in 260, and when danger is to be apprehended from vehicles running back, a catch-siding with points weighted for the siding, or a throw-off switch, to be provided to intercept runaway vehicles at a distance outside the home-signal for the ascending line, greater than the length of the longest train running upon the line. Under similar circumstances, when the line is single, provision for averting danger from runaway vehicles to be made-- (1) At a station in one of the following manners:-- (_a_) A second line to be laid down, a second platform to be constructed, and a catch-siding or throw-off switch to be provided on the ascending line inside the loop-points. (_b_) A loop-line to be constructed lower down the incline than the station platform with a similarly placed catch-siding or throw-off switch. (2) At a siding-junction in one of the following manners, except where 27 it is possible to work the traffic with the engine at the lower end of a goods or mineral train, in which case an undertaking (see No. 35) to do so, given by the company, will be accepted as sufficient:-- (_a_) A similar loop to be constructed as in the case of a station. (_b_) Means to be provided for placing the whole train on sidings clear of the main line before any shunting operations are commenced. 17. Engine-turntables of sufficient diameter to enable the longest engines and tenders in use on the line to be turned without being uncoupled to be erected at terminal stations and at junctions and other places at which the engines require to be turned, except in cases of short lines not exceeding 15 miles in length, where the stations are not at a greater distance than 3 miles apart, and the railway company gives an undertaking (see No. 35) to stop all trains at all stations. Care to be taken to keep all turntables at safe distances from the adjacent lines of rails, so that engines, waggons, or carriages, when being turned, may not foul other lines or endanger the traffic upon them. 18. Cast-iron must not be used for railway under-bridges, except in the form of arched-ribbed girders, where the material is in compression. In a cast-iron arched bridge, or in the cast-iron girders of an over-bridge, the breaking weight of the girders not to be less than three times the permanent load due to the weight of the superstructure, added to six times the greatest moving load that can be brought upon it.