Train Control

The majority of the methods that are used to control model trains fall into two general categories.  These are called cab control and command control.
    Cab control is the most common system, and for the most part the most simple.  There are very complex cab control layouts out in the world, but your starter set with a loop of track and a power pack is the simplest form of this system.  Cab control means that one or more power packs are used to control one or more sections of track.  All the locomotives on a particular section of track are controlled together by a power pack (or cab) that is connected to that section of track.  With a series of toggle switches, one locomotive can run the whole layout while being controlled by the same power pack.  Many HO layouts in the Atlas books that are available in hobby shops use cab control, and the wiring is laid out so that the builder can easily understand it.  In fact, Atlas makes specific components to promote this type of wiring.  I have cab control on my layout, and have three cabs that are connected to three power supplies, which allow me to run two main lines and one yard at the same time.  Of course it takes a lot of switch throwing to get a train made up in the yard and out on the main line, drop off and pick up cars at various industrial sidings, and get the train back into the yard and broken down.  For a meet, the main lines can both be set for continuous running.
    Cab control has the advantage of simplicity and low cost.  You do not need any fancy electronics to make it work, nor do you have to make modifications to your locomotives.  Troubleshooting is a relatively easy task.
    Cab control does have serious disadvantages.  One is that different trains on a single section of track will respond to the same control commands from your throttle.  This will definitely limit operations and flexibility.  The second problem is that control of your locomotive requires a great deal of switch flipping which can prevent you from enjoying running your trains.
    Command control gets around the two problems of cab control.  It involves circuitry that allows engine control commands to be sent directly to a particular locomotive independently of all other locomotives on the same track.  Each locomotive can run over the layout without worrying about flipping cab switches.  Individual trains can run at different speeds or even different directions on the layout without regard to other trains. (Yes, cornfield meets are now possible).
    A common feature of command control is that each locomotive carries some sort of command receiver or decoder that controls the motor functions.  This receiver is dedicated to a particular locomotive, and will only respond when commands are addressed to it.  This adds a certain amount of electronic complexity that is not found in cab control.   Even though command control and its components are somewhat complex, the various manufacturers have done a fairly good job of making their systems installable by even those who consider themselves "technically challenged".
    Many people combine command control with battery power.  This allows a degree of freedom not possible with any track powered trains.  You can use less expensive track which never needs cleaning as conductivity is not a requirement.  Battery power carries some liabilities which are considered serious by some.  First, a fairly large battery is required.  It can be placed inside some locomotives, but others require the battery to be carried in a trail car.  Batteries have a limited energy storage capability and must be recharged.  Typical battery run times vary, and it is difficult to MU locomotives to share the load properly.  Batteries do not last forever, and need to be replaced on occasion.  Heavy power draws for lighting, smoke, and sound systems will considerably reduce battery run times.
In all, battery power with some form of command control is a very successful system.  Many operators that have converted to battery do not seem inclined to convert back.
    Track powered command control also has advantages and disadvantages.  With track power available, locomotives can run continuously with long heavy trains, and the accessory functions for sound, smoke, and lighting have no effect.  With some track powered command control systems, multiple unit control is easy and very effective.  In this case, it works the same as prototype MU diesel control.  In this case the engineer has all the locomotives under control of his throttle.  Speed control and power sharing between locomotives is handled automatically.
    Track power requires that the track be in good condition and at least reasonably clean.  Continuity between joints is a must, as it is with any track powered system.  You do not, however, have to run a lot of wiring to various blocks and switches, since power is on the track at all times.
    There are many systems available on the market to provide DCC.  Everyone has their favorite, and it would be difficult to review each one.  All these systems provide control ranges of 50 to 100 feet in the best case.  Each system is manufacturer specific, and is not compatible with other systems.  LGB, MTH, and USA are prime examples of manufacturer specific systems.  Because of the incompatibility of the technologies used, it is impossible to mix and match locomotives from these manufacturers and expect to be able to control them using one command system.  If you run different manufacturer's locomotives on your layout, it would appear best to go with a system like RCS, Locolinc, Aristo TE, Airwire, Lentz, etc.  This way all your locomotives will be equipped with the same system, and can be run from one command controller.   I would recommend that you go to each manufacturer's web site and
get as much information as possible before making your choice. 
    Command control systems are not cheap.  The up front cost is high, since you are buying the control system (computer?) and get one decoder and controller.  Adding decoders can be done at reasonable cost, and additional controllers are also available.  One controller can control a large number of decoders, but it means that only one person can run trains, so additional controllers are advised if you are going to want to share the fun.    Make sure that you buy a DCC system that will run your Sierra or Phoenix sound systems if you have them installed.  The LGB, MTH, and USA systems only work with their own proprietary manufacturer provided sound modules.