Fig. 506 shows the arrangement applied to a double line with the outlying siding turning out of the UP main line, the points lying in a trailing direction for the running trains. Before the special and _only_ key can be withdrawn from its seat in the interlocking frame of the signal cabin, all the UP main line signals must be set to _danger_, and cannot be moved from _danger_ until the key is restored to its proper seat again. When the key is removed from the signal cabin, it can be taken to the ground frame at A, inserted in the key opening, and by turning it partly round, will release the bar which locks the levers of the facing point bolt-lock and the points. When these two levers are free the points can be opened, and vehicles moved into or out of the siding B C, but the special key cannot be withdrawn from the ground-frame A, until the points and facing point bolt-lock are put back again into their normal position for main line working. When the operations at the siding are completed, the special key can be removed, and taken back to its proper place in the signal-cabin, and ordinary working be resumed. Fig. 507 shows the application of the detached locks on a single line, and is a sketch of a portion of railway on which there is a small station B, with a goods siding F G, where the traffic is too small to require anything more than ground frames and detached locks. An engine-driver before leaving the station A, receives a train staff, which gives him possession of the line as far as C, including of course the intermediate station B, and this staff he must carry with him and hand over to the signalman on his arrival at the end of the section at C. At each of the points D and E is placed a two lever ground frame, similar to the one shown in Fig. 506, and attached to the train staff is a key, which will operate either of the two ground frames, but only one at a time, as the key must be inserted before the levers can be moved. When the train is proceeding in the direction from A to C, it will be more convenient to shunt vehicles into or out of the siding F G, by means of the points E, but when proceeding from C to A, the points D will be more convenient. Whichever of the points 334 be used, they must be set, and bolt locked for the main line before the train staff and its key can be withdrawn from the ground frame and restored to the engine-driver. As the siding is _trapped_ at F and G, it is impossible for any vehicles to be moved out on to the main line except through the medium of the train staff and key. The same arrangement of detached lock is equally available for a single siding with only one set of points.