Power / DCC / Electrical
If you found yourself on this page and are wondering what the difference is between AC, DC, and DCC, this may help.
DC (Direct Current) is the conventional method of powering most N, HO, On30, and G Scale trains. The power pack supplies varying levels of voltage to the rails which, in turn, changes the speed of the locomotive. To change direction, the polarity on the rails is simply reversed. This works reliably and is fine for less complicated layouts and with few trains. However, since you are really controlling power to the rails, any trains on the same rails will go in the same direction and cannot be controlled independently (ie - one cannot be stopped while the other is moving). To work around this, track blocks can be created so that power to these isolated sections of track can be independently controlled. This can lead to some complicated wiring and some confusing operation. However, it can and is done.
This is a newer approach to model railroading and in a nutshell, works like this. The concept of DCC (Digital Command Control) is that you are now directly and independently controlling individual locomotives anywhere on the layout. To make this work, each locomotive must have an installed computer chip known as a decoder. In contrast to how DC works, in DCC a constant voltage is applied to the rails at all times. To make an engine move, your throttle and command station are used to send digital instructions via the rails to your locomotives. Some of the benefits are that you don't need to worry about what block the locomotive is occupying, locomotives can act independently of one another, and with most decoder equipped locos, you can turn lights on and off. Many are now also coming with sound capabilities that can all be controlled independently. Because you don't need all of those blocks, wiring also becomes simpler. Keep in mind that you cannot control DC engines on a DCC layout.
Now, many O Scale layouts work on a 3-Rail system that is AC powered. AC (Alternating Current) is different from DC in that the direction of current quickly changes direction at a high frequency. Because of this, motors designed to be used on AC will not work with DC. They need a different power supply. Bachmann, Lionel, and MTH are examples of trains that require AC Power supplies.