This is "ACSR": aluminum cable, steel reinforced. Within a certain number of miles of the ocean, you can't use aluminum, period, because of the corrosive action of the chlorine in the salt air; everything must be copper (and brass/bronze). Aluminum trolley wire wouldn't work because aluminum is too soft, and copper trolley wire is medium hard-drawn, not soft-drawn, so that it won't stretch when tensioned, is hard enough to resist wheel and shoe wear, but is still workable by human hands. Some of the NH and PRR trolley wire was phosphor-bronze, which is almost impossible to bend wihtout a serious set of power tools.