Answer:

The optimal track voltage is 13 volts. However all TCS decoders will safely operate up to 24 volts track power.  It is important to know how to measure track voltage. Please carefully read the following.

The best way to measure the track voltage is to connect a volt meter set to measure DC volts between the blue wire and the white wire of a decoder. With the meter hooked up, turn on the headlight (function zero). You will then get an accurate reading. Some command stations have a means of adjusting the track voltage. Easy DCC is one that does. Adjust the track voltage to obtain 12 volts between the blue and white wires. Track voltage is typically about 1 volt higher then what is measured between the blue and white wires. If you measure 12 volts between blue and white you know your actual track voltage is 13 volts.

Important! Do no try to read track volts directly with a digital meter. You will not get an accurate reading. This is because the track voltage is AC voltage at 10khz frequency.  Most meters are not designed to accurately read AC voltage with frequencies above 1khz.  They will typically give you a reading less then actual voltage! This is true of even good meters (Flukes, Beckmans etc.)  By measuring DC volts between the blue and white wires of the decoder with the button zero (function zero) on you are sure to get a accurate reading.