This is heavy Gauge 12 AWG or so common or earth wire that is used to assist in establishing a common zero voltage across several Digitrax items. In this particular case I have a BDL168 block detector by Digitrax along with their power manager PM42 all requiring common wiring to the booster and command station via a common wire. I use earthing cable not dissimilar to household wiring cable for this purpose as it bends to shape and stays in shape and will have no problem transmitting current given it's high cross sectional area and relatively low resistativity.