If you want to accurately measure power output (both amps and volts) AT THE TRACK, by far the easiest and most accurate way for the non-electrical engineers among us is to buy the DCC Specialties RRAmpmeter II. It's about $100 at most e-tailers, but if you want accurate information about what's going on AT THE TRACK with your DCC system, this is the way to go.
John C.