Security of interlocking systems for railroad track switching

Compares various interlocking systems-mechanical, electromechanical, relay, and geographic-in connection with route establishment, route security, signal control, and establishing the normal positions. Particular attention is also given to the circumstances that might impair safety and the steps taken to deal with such conditions in each type of interlocking system. The security comparisons are given in some detail. It is concluded that mechanical interlocking security is based on careful maintenance, that security must be automatically guaranteed, that the interlocking system alone should guarantee security, that modern relay techniques are adequate, that relay testing is to detect conductor faults, that signal relays must have all contacts of the opposite condition open as long as one contact of the other relay condition is closed, and that geographic interlocking provides reliable and unprecedented security.