The first thing you need to decide in designing a layout is the power system.
Traditionally, model railroads were powered through the tracks using DC (direct current.) This system is still used on some garden railroads. Aristocraft has a system that uses DC track power with a walk-around radio throttle. This works fine if you only run a single train, but it becomes more complicated with two or more trains. To do this, you need to divide the layout into power blocks. Then every train in the same block must run in the same direction, run at the same speed, and stop at the same time. If you want to run more than one train at a time, then the better route is to use digital command control, battery power, or some comparable system. All of these approaches involve installing some electronics in each of your locomotives. This increases the cost of each locomotive and requires some work to do the installation, but the improved flexibility in operations is tremendous.
Digital command control (DCC) uses track power, but the power is about 20 volts AC (alternating current) instead of DC. The power pick-ups in each locomotive are isolated from the motor and a small circuit board called a decoder is installed between the power pick-ups and the motor. This circuit board feeds DC power to the motor, controlling the direction of travel and the speed. A DCC command station inserts a digital signal into the track current which the decoders can detect. This signal allows each decoder to receive individual instructions which control the direction and speed of the locomotive. DCC can also be used to control a sound system and lights on a locomotive or to consist multiple locomotives to run together pulling a single train. Radio throttles can be used with DCC. The throttles send radio signals to a small receiver attached to the command station. Each throttle can be set to control a different decoder, and hence a different locomotive.
With battery power, special cars are filled with powerful batteries and these are charged before each operating session. Each locomotive must be provided with a battery car. If the batteries run down, another car with freshly charged batteries can be connected to the locomotive. Components that do the job of a DCC decoder and a radio receiver are installed in each locomotive. Radio throttles are then used to control the locomotives. Battery power avoids the need to keep the track clean enough to allow power pick-up from the tracks, but this is a trade-off since it requires battery cars and battery charging. And you still have to clean the tracks of any debris large enough to cause derailments. Another advantage of battery power is that you can use less expensive aluminum track. Aluminum track does not conduct electricity well enough to use for track-powered layouts. Another disadvantage of battery power for operations is that you can't pick up or drop off the battery car unless you exchange it for another battery car. So one car (the battery car) stays on the train behind the locomotive at all times.
One last options is live steam. Garden railroad locomotives are large enough to be equipped with working burners, boilers, and cylinders. Live steam locomotives can be controlled using manual controls in the locomotive or using radio controlled systems. Nothing could be more realistic than a live steam locomotive, but you have to replenish water and fuel (butane is often used) from time to time. A growing number of garden railroaders run live steam. If you are going to run sessions with young operators, you should consider the danger of burns with live steam.
In some form or other, these are about all the options there are for power: DC track power, DCC or some other system that combines track power with control of locomotives through electronics installed in each locomotive, battery power with control of locomotives through electronics installed in each locomotive, and live steam power with manual controls or control of locomotives through electronics installed in each locomotive.
On the Lake Town & Shire, we use one 8 amp Digitrax DCC command station, two Digitrax boosters, and Digitrax radio throttles. We have used the same command station since we began operations in 2000 and we have been very happy with it. A few years ago, we divided the layout into three power sections. One runs off the command station as before. Each of the other two runs off a separate booster. This allows two sections to continue operating if one section develops problems such as a faulty rail connector. This also makes it easier for us to diagnose any track problems more quickly. Our command station is powerful enough to run four or five locomotives at one time even before we added the two boosters. We use mostly Digitrax decoders, although we have used a few Quantum decoders with sound.
I recommend DCC, particularly the Digitrax system since we have had good experience with it. However, other systems, including battery power, can work well for operating sessions. I would not recommend DC. I used DC on indoor HO and N scale layouts thirty years ago and I would not want to try it in the garden unless I was convinced I would never want to run more than one train at a time. I also would not recommend live steam for operation sessions, although I think it would be great to have a live steam engine to run on our layout when we are not doing operations. If you do decide to use DCC, you might want to take a look at the Digitrax website. An online supplier I have used for DCC is Southern Digital. Owner Paul Lator has been very helpful and his prices are competitive.