Looking at the Gardner Bender(GB) website, instructions in using this multimeter should be available in pdf format also do a general search on the web for using a multimeter on AC model trains. Check out trainelectronics.com/METER_Workshop/ for meter use,this should be beneficial, the ability to use a multimeter and continuity tester are essential in troubleshooting AC and DC model trains.
The referenced above website is for DC trains, 3-rail is AC voltage, outside rails are negative(common) inside rail is positive(hot) from the transformer. Same with engines and lite caboose and passenger cars, the center roller is positive(hot), outside wheels are common(negative).
