A few comments:
1. While you can run LEDS on track power or other AC source, it will significantly shorten their life. You should use a diode and a resistor in series with the LED. For up to 18V track power, a standard 1N4003 diode and a 470 ohm 1/4W resistor will handle one standard LED. That will run 10 mA through the LED. You can go brighter (up to 20 mA) with a lower value resistor.
2. Online LED calculators will tell you what ohm resistor as well as the wattage rating needed to dissipate the heat generated. These calculators are very useful, but they assume DC voltage. It gets complicated when you are rectifying AC with a diode. 18v AC effectively produces only 8v DC (14VAC gives 5.6V DC, and 10VAC gives 3.8V DC). Here is a "math-fun" discussion of doing the rectified DC voltage calculations. It also shows the Diode/Resistor/LED circuit.
To use the LED calculator, you plug in the supply voltage (DC), voltage rating of the LED (they have hints to help you determine that), current you want to supply the LED (usually 10-20 mA depending on brightness desired), and number of LEDs in series in the circuit.
Bob
