The purpose of DCC is to allow you to control the speed and direction of travel of your locomotive on your layout. There are four different ways that one can accomplish train control.
You will find people talking about "conventional" control. What they mean by that is using simple AC or DC to control your locomotive. AC stands for alternating current, where the voltage on the rail alternates between the plus and minus values of some voltage. The amount of voltage is controlled by a transformer that converts the electricity coming into your house into a lower and safer voltage to be applied to the rails of your layout. The wheels of the locomotive pick up this voltage and route it to the internal motor (or motors) in the locomotive. Using AC voltage forces the magnets in the motor to be rotated, which in turn rotates the axle coming out of the motor, thereby moving the wheels.
DC stands for direct current. This offers a steady voltage to the rail. The amount of voltage is also controlled by a transformer, but instead of the voltage constantly swinging back and forth between the plus and minus values, it remains at a steady positive value. This has several advantages over AC, including the ability to have smaller motors and more efficient use of the available electricity.
Because of the fundamental differences in the way motors are constructed, you cannot have a locomotive with an AC-equipped motor operate on a DC-powered layout, nor a DC-equipped motor operate on an AC-powered layout. Some electronic components can be added to make this possible, but "out of the box", no, you cannot do that.
Both AC and DC are extremely simple to hook up to your train layout. The transformer will have two connections, and you simply connect one wire to one rail and the other wire to the other rail. The transformer will have some sort of knob or controller on it to regulate the amount of voltage that is to be applied to the rail. This is what makes the locomotive move. They typically also have a directional switch that allows you to move the engine forward or in reverse. If all you have is a simple loop of track, e.g. a Christmas tree layout, than that is all you need. However, there are limitations. For example, all the locomotives you put on that track are run at about the same speed and in the same direction, similar to synchronized swimmers. You cannot control each locomotive independently.
This is where "Digital Command Control" (DCC) comes in. It is an NMRA-recognized standard for controlling the movement and behavior of model trains.
Briefly, the fourth method for controlling your locomotives on your layout is via battery power, which we fully describe in this article.
DCC embeds a digital signal into the track power that is connected to the rails of a DCC-equipped layout. All rail connected to this system has full power (typically 12 volts) applied to it at all times.
Locomotives that are equipped to run on a DCC-based layout, have a circuit board installed in them (called a "decoder", because it decodes that digital signal). This circuit board is programmed to only respond to one particular set of instructions matching some internal number. We typically tend to use the engine's road number, for ease of use.
When the layout operator sends a message to the DCC layout via a throttle, the message includes that specific number. Only the decoder in the engine that had been previously programmed to respond to that number, will respond to that message.
DCC allows for controlling the locomotives' speed, its direction of travel, turning on or off the lights, making some lights have special effects, turning off and on sounds (if so equipped), and making an almost infinite number of sounds. All this can be controlled from the throttle, which can be a hard-wired, fixed system, a wireless throttle, or even a local wi-fi-connected cell phone or tablet.
If you have a simple layout, or if you are about to build a layout anew, switching from DC (or AC) to DCC is not that difficult. However, if you have an existing layout, especially a complex one, DCC does require some changes. Most modelers who wanted to built a layout where they could have independent control over multiple engines on their layout, divided the layout up into blocks. Each block was controlled by one DC (or AC) transformer. When an engine travelled into a block, it became controlled by that block's transformer. This allowed several operators to run the layout independently, such as a switcher working in a freight yard working independently of a mainline train moving passengers.
When switching from DC (or AC) to DCC, you no longer need to those blocks, and the special kind of wiring that those blocks required. It really does simplify your layout's wiring. You could have your entire layout be basically one large "block".
However, DCC systems do have an upper limit to the amount of current that they can supply. Because you can run more than one locomotive on the layout at a time (as opposed to the DC block where only one locomotive was controlled at a time, typically), it is possible to have so many engines running so as to exceed the current limit of the DCC system. Sound-equipped engines, for example, can draw a lot more current than non-sound engines. In the case of S-scale, modern locomotives draw a lot less current than older, say 1950s, engines, unless they have been reworked.
So, if you have an existing, large layout, you may still need to keep blocks (in DCC parlance they are called "districts"), with each district being powered by a DCC component.
With most DCC systems you will have a "command station", which is the main brains behind the whole system, and it will typically also contain a power supply, so that you can use that as the one and only major component that you need to run your small layout (except for perhaps a throttle or two). For larger layouts where you plan to have districts, each district can be powered by a separate "booster unit", which will be the power supply for that district. The main unit and the booster units will communicate with each other via an extra wired connection, so that they can seamless transfer the commands the throttle sends to an engine as it passes from one district to another.
Within S-scale, there are three different ways that engines can be configured. Original A.C. Gilbert American Flyer engines can be set up to need AC (alternating current). Some Lionel engines were that way as well. More modern engines will likely be set up to use DC current. The third possibility is that the engine is already factory-equipped with the electronics to allow for operation on a DCC layout (some of those, such as MTH's and Lionel's engines may even detect what type of system they are being run on, and respond appropriately and automatically).
AC and DCC don't get along, unless some sort of conversion circuit is created. So, if you are planning on doing the conversion to DCC yourself, it is best to consider replacing the motor in the AC locomotive to one that is DC. You can do that yourself or have someone do it for you for a fee, plus the cost of a new motor. A newer DC motor is likely to draw less current and perform better anyway, so that will be a bonus.
Once your locomotive has a DC motor in it, it is a candidate for DCC conversion. A locomotive that came from the factory as a DC locomotive (or one that was converted from AC to DC), and so it runs on a simple DC transformer, will have pick-ups somewhere (wipers on the back or tops of the metal wheels, or a separate sliding shoe) to pick up the DC current. These wipers are directly connected to the motor. You will need to keep those wipers, but you must remove their connecting wires from the motor. Important: there must be NO direct connection between the wheel/rail wipers and the motor for a DCC installation!
You will need to purchase a DCC decoder and be able to install it into the locomotive (or have someone do that for you). Because DCC is an international standard now, you should be able to buy the decoder of choice from any manufacturer, regardless of who the manufacturer of your DCC system is. Finally, the decoder must be installed into your locomotive, in between the rail pickups and the motor (and lights and sound system, if so equipped).
Lionel created their "Trainmaster Command Control" (TMCC) in 1995. Famous musician, Neil Young, part owner of the company at the time, helped to create this system. Lionel started offering licenses to use and manufacture TMCC decoders to other companies starting in 2000. While it uses the same command codes as standard DCC, they use a 455kHz radio signal to carry the command codes rather than embed them in the track power, and they require that the track power is AC, not DC. The signals are sent from the throttle to their "Command Base", which then forward it to the appropriate engine. Non-TMCC locomotives can be operated on the same layout on which TMCC-equipped locomotives are run. Lionel stopped selling the TMCC system in 2010.
In 2007, Lionel introduced their "Lionel Legacy" system (it is an odd name for a "newer" system to call it "legacy"). It is backward-compatible with TMCC-equipped locomotives. It also uses the ability to send control signals via radio, and they are not embedded in the rails. Lionel Legacy is not compatible with standard DCC, and, so far, Lionel has not published or licensed their Legacy command codes. The Legacy system cannot operate non-Legacy/non-TMCC engines, without purchasing additional components.
Lionel has a YouTube video explaining the differences and similarities between their TMCC and Legacy systems.
Lionel's "LionChief" and "LionChief Plus" is a method of controlling an engine via remote control only. The power comes from the track (a constant voltage to the rails) and the decoder in the locomotive responds to the commands sent via the throttle or remote-control. This system is not compatible with their TMCC or Legacy systems. The LionChief systems are sold with the engine itself. You can kind of think about this as buying a television and getting a dedicated remote-control for that TV. The LionChief Plus is an upgraded system from the original LionChief; it offers more and better features. LionChief-equipped locomotives can be run on any layout that has constant voltage (such as a layout that is current based on TMCC or Legacy), but the LionChief remote control is required to actually operate the LionChief-equipped engine. A LionChief Plus-equipped engine can be run in the same manner on the same type of layout, but you can also switch it such that it is run as a conventional engine (i.e. controlled by just a transformer). In that manner, though, not all of the features are available.
MTH Electric Trains uses their Digital Control System (DCS), which they first introduced in 2002. The DCS system transmits the control signals via the rails, much like a standard DCC system does. The MTH DCS base unit is called a "Track Interface Unit" or TIU. This is similar to standard DCC's term of "command station". The DCS system allows for bi-directional communication between the locomotive's decoder and the main system, and it separates the signal from the rail power. The command signals are transmitted at 10.7MHz. MTH's "Protosound" systems are sound-enabled decoders that are compatible with the DCS system as well as with standard DCC systems.
The Lionel TMCC system will not control MTH DCS-equipped engines. However, the MTH DCS system will operate TMCC-equipped models by means of an interface cable that connects the Lionel CB-1 command base to the DCS Track Interface Unit. DCS can coexist on the same track at the same time with either Lionel TMCC or Legacy command systems. Engines with either system can be operated simultaneously as long as both command control units are installed on the track. Inter-operability is not guaranteed, and so some features may not work.