A system for the control of railroad trains so as to avoid collisions. Block stations are placed a few miles apart, connected by telegraph lines, and provided with signal boards or lights. The rule is that no train shall pass a block station while a train is on the section in advance, and until word comes back that the section is clear. Thus, if operated perfectly, there can be only one train on a section of three or four miles at a time, and collisions would be impossible. But men are not always to be trusted, and an automatic block system, in which the trains themselves work the signals, through electric attachments, is being introduced. The block system was first introduced in 1851, and is now much more common in Europe than in the United States.