I haven't seen one in a long time. Does it have an on/off switch? Does it have a 110 volt plug? I don't remember if it runs off household current or if it picks up power from your power pack. You do not want it on all of the time. It uses high frequency a/c to burn off dirt and corrosion. The high frequency a/c will sometimes fry can motors.
If it plugs in to household current, you would plug it in and hook up the low voltage wires to your track. If you have an old Athearn locomotive with one of their open frame motors, run it around the layout with the Relco turned on. Make sure the locomotive runs on every single piece of powered track. The track, the locomotive wheels, and the commutator will be cleaned. Once the cleaning is done, either turn the unit off, or disconnect it from the layout. If it doesn't use house current to operate, it will work off the a/c termonals of your power pack. Again hook up the input wires to the a/c terminals and the output wires to the track. I think they should be marked or else the point of origin of the wires will be marked. Then you just run your old Athearn open frame locomotive all over the layout.
It is very important to either unhook the Relco unit from the track or shut it off before operating the layout if you have locomotives with can motors.
If you don't have an old Athearn locomotive woth an open frame motor, they are available inexpensively used at train shows, swap meets, etc. You may even find some available here on the Gauge "Buy & Sell" forum.