Most electrification systems use overhead wires, but third rail is an option up to about 1,200 V. <answer>Third rail</answer> systems exclusively use DC distribution. The use of AC is not feasible because the dimensions of a third rail are physically very large compared with the skin depth that the alternating current penetrates to (0.3 millimetres or 0.012 inches) in a steel rail). This effect makes the resistance per unit length unacceptably high compared with the use of DC. <answer>Third rail</answer> is more compact than overhead wires and can be used in smaller-diameter tunnels, an important factor for subway systems.
What is physically more compact tird rail or overhead wires?
Third rail