A double-pole, double-throw relay with actuating mechanism driven by a momentary-power, twin coil solenoid. (A solenoid is a cylindrical coil of insulated wire in which an iron core is made to move back and forth by a flow of electric current.) With just a few simple connections, this small device, which may be mounted anywhere, can:

Power the frogs on Atlas' Mark 3 & Code 83 #6 turnouts
Control trackside signal lamps to show turnout point positions
Operate control panel lamps to show settings of remotely located turnouts (switches)
Provide selective track siding control. Sidings are powered only when points are aligned for siding. Siding remains dead when points are set for the mainline.
Control X-sections automatically. This very useful track arrangement is thoroughly discussed in Atlas' Complete Wiring Book.