Figs. 504 and 505 show plan views of four levers in a signal cabin 332 taken just above the level of the tappets. In Fig. 504, all the levers are in their _normal_ or forward position, with the home and distant signals at _danger_, and the facing points leading into loop or siding lying for main line. Previous to the approach of a train on the main line, the home and distant signals have to be lowered, and will require the pulling over of levers 1 and 2; but these levers cannot of themselves be moved, as the wedges P and Q are locked by the straight side of lever 3. The operation would therefore be as follows:--points lever 4 being set in its normal position for the main line would remain forward, lever 3 working the facing point bolt-lock would be pulled over, and in doing so would move the wedge R to the right into the recess of tappet of lever 4, locking that lever, and presenting the recess of its own tappet ready to receive the wedge Q. Lever 2 can then be pulled over, and will move the wedge Q to the right into the recess of tappet of lever 3, and present its own recess for wedge P. The pulling over of lever 1 completes the series, by moving the wedge P over to the right into the recess of tappet of lever 2. Fig. 505 shows the positions of the tappets and wedges with the levers 1, 2, and 3, pulled over to make the combination described. Upon examination, it will be seen that levers 2, 3, and 4, are all securely locked, the points cannot be moved, nor the facing point bolt-lock withdrawn, nor the home signal changed until the lever 1 is pushed over again into its normal or _danger_ position. To restore the levers to their forward position, they must be set back in the reverse order to which they were pulled over. To simplify the explanation, only four levers are shown in the above sketches, but the principle is constantly extended out to a very large number of levers, and in many cases necessitates the introduction of several rows of wedges as indicated by the dotted lines. In some instances a combination is effected by pulling a certain lever only half over. In some systems the preliminary action or spring handle locking is adopted, in which the locking is actuated by the small side lever, similar to the one marked M on Fig. 503. The advocates of this arrangement claim increased security and precision in the interlocking, while on the other hand it is alleged that the mechanism is rendered more complicated without any corresponding advantage. Detached Lock.--Sometimes there is in the vicinity of a railway station, 333 a siding which is too far away to be worked direct from a signal cabin, and not sufficiently used to warrant a separate cabin. Such sidings can be worked by a small ground frame opened or locked by a special key attached to the interlocking machinery in the adjoining signal cabin on a double-line railway, or attached to the train staff on a single line.