BTW, I'm assuming you would be using a digital meter. That being the case you would not have to worry about the polarity of the meter leads. If you're using an analog meter that goes from zero to positive (as opposed to a center zero -0+), you will. If you don't know the polarity of an analog meter, the best thing is to hook it up, put an engine on the track and just barely turn the power pack on and see which way the meter needle goes. If it goes up, you're good to go, if it goes backwards, reverse the two meter leads before you go any further.
You can get a really cheap digital meter that will do the job at Harbor Freight. They are selling a small pocket-sized meter for $2.99 at our local store and it measures AC and DC volts, AC and DC amps and resistance.