Any lighting project begins with choosing the bulbs. There are many types of miniature lights out there, and most of them look very similar at first glance. LED's (Light Emitting Diodes) are becoming an increasingly popular lighting for everything from models to traffic lights. LED's offer great advantages in efficiency and bulb life over incandescents. Their popularity should continue to grow in coming years.
Conventional incandescent bulbs have been around for decades and remain a popular choice for their cost and simplicity. While the color of LED's has improved, there is still no better way to reproduce the warmth of traditional lighting than with an incandescent. These bulbs come in several sizes and most importantly, voltages. If you are replacing an existing bulb, be sure to replace with the same voltage. For new projects, choose a bulb that will fit in the space and system you have.
The most common voltages for miniature bulbs are 1.5, 3, 10-12, and 14. Put a 14 volt bulb on a 1.5 volt circuit and it won't glow very bright (if at all.) A 1.5 volt bulb on a 12 volt circuit won't last but a few nano-seconds.
Generally, the smallest bulb sizes are available in only lower voltages. There are ways to compensate for the differences. Resistors can be used in conjunction with smaller bulbs. You can also wire pairs of bulbs in series (connect the lead of one bulb directly to the next.) By doing this, each bulb will see only 1/2 the voltage.
You can increase the life of your bulbs by operating them on less than their full rating. For example, run a 14 volt bulb on 12 volts. The light will not be as intense, but should last much longer. This is an important consideration when working inside buildings, locomotives or cars where lights are not easy to change. It is also easier to achieve these results with higher-voltage bulbs.
Bulbs come in many shapes and sizes. The smallest are "grain of rice" and "grain of wheat." These are useful in small scales, and in light fixtures where the bulb may be seen. For hidden interior lights, larger bulbs are a more inexpensive and efficient option.
Incandescent bulbs can operate on either AC or DC power. You can power bulbs directly off of track power (if their voltage rating is high enough) but this reduces the power available to your trains. A seperate power supply is recommended.
Many train set supplies have AC terminals for lights and accessories. Check the output of this prior to attaching bulbs. They are typically on the high end and can not be regulated. A regulated power supply can be found for only a few dollars and provide a consistent voltage to all of the lights on your layout. Most bulbs only draw a few milliamps of power.
Run a bus wire beneath the layout to power your lights. As with track wiring, color coding will help keep things straight. Because current draw is so low, lighting buses do not need to be a heavy wire gauge. An 18 to 20 gauge bus ought to be more than sufficient.