The electric train-staff may also be used in a permissive block system, the section of the track between stations _A_ and _X_ constituting the block. In Fig. 19, showing the machine, a horizontal arm is seen to extend across its face and to the right. This is the permissive attachment which must be operated by the special staff shown on the left half of the machine about midway of its height. If it is desired to run two or three trains or two or three sections of the same train from _A_ before admitting a train at _X_ in the opposite direction, the operator at _A_ so advises the operator at _X_. The latter then permits _A_ to remove the special staff with which the extreme right-hand end of the permissive attachment is unlocked and a tablet taken out. This tablet is equivalent to a train order and is given to the train immediately starting from _A_. A second tablet is given in a similar manner to the second section or train, and a third to the third section. The last section of train or train itself starting from _A_ takes all the remaining tablets and the special staff for insertion in the machine at _X_. In this manner head-to-head collisions are prevented when a number of trains are passing through the block in the same direction before the entrance of a train in the opposite direction. This system has been found to work satisfactorily where it has been used in this country, although its use has been quite limited. Evidently, in itself, it is not sufficient to prevent rear-end collisions in a block between trains moving in the same direction. In order to avoid such collisions where a train falls behind its schedule time or for any reason is stopped in a block, prompt use must be made of rear flagmen or other means to stop or to control the movement of the first following train. [Illustration: FIG. 20.] The most improved form of high-speed train-staff machine is shown in Fig. 20, as made and installed by the Union Switch and Signal Company and used by a number of the largest railroad systems of the United States. In these machines the staffs are but a few ounces in weight. CHAPTER XXIII.