DCC

Digital Command Control (DCC) is a standard that many manufacturers follow to allow the independent control of multiple locomotives and accessories, simultaneously, on a layout. This standard is defined by the National Model Railroad Addociation (NMRA). This overcomes the issues of controlling multiple trains using standard DC. See Digital Command Control Advantages Over Direct Current for additional benefits of using DCC over DC. 
Digital Command Control uses a purely digital signal with no analog component on the rails. The locomotive mounted decoder or track connected decoders are purely digital and respond to the DCC messages.
The DCC protocol is the subject of two standards published by the NMRA: , and . Several recommended practices documents are also available.
==Overview of Digital Command Control==
The system consists of power supplies, command stations, boosters and decoders.
The brain of the layout is called a command station, which creates the digital packet. The majority if systems only support a single command station. Many command stations are integrated with an amplifier (booster) which, in combination with its power supply, modulates the voltage on the track to encode digital messages while providing electric power. For large systems additional boosters may be employed to provide extra power.
The voltage to the track is a pure digital signal. The DCC signal does not follow a sine wave as it is not an AC waveform. The command station/booster quickly switches the voltage on the rails on and off, resulting in a modulated pulse wave. One rail is always the inverse of the other, with each data pulse repeated. The length of time the voltage is applied provides the method for encoding data. To represent a binary one, the time is short (nominally 58 µs), while a zero is represented by a longer period (nominally at least 100 µs). As there is no polarity, the direction of travel is independent of the rail's phase.
Each locomotive is equipped with a [Multifunction_Decoder|multifunction decoder]] that takes the signals from the track and, after rectification, routes power to the electric motor as requested by a throttle or computer. Each decoder is given a unique address for the layout, and will only act on commands intended for themselves. This provides independent control of locomotives anywhere on the layout, without special wiring requirements. Power can also be routed to lights, smoke generators, and sound generators. These extra functions can be operated remotely from a throttle. Accessory decoders can also receive commands from the controller in a similar way to allow control of turnouts, uncouplers, other operating accessories (such as station announcements), and lights.
In a segment of DCC-powered track, it is possible to power a single analog model locomotive by itself (or in addition to the DCC equipped engines). The technique is known as zero stretching. Either the high or the low pulse of the zero bits can be extended to make the average voltage (and thus the current) either forward or reverse. However, because the raw power contains a large harmonic component, DC motors heat up much more quickly than they would on DC power, and some motor types (particularly coreless electric motors) can be damaged by a DCC signal. Coreless motors should never be used without a decoder installed as they will quickly burn up.
See Introduction to DCC to get started with DCC.
==DCC Future==
The NMRA has defined Digital Command Control through a number of published standards, allowing room for future expansion and enhancement in the DCC Standard.