electric cable must be absolutely level, otherwise the frame connecting to it jumps up and down as the train moves. one cannot stretch the cable so tight that it will be absolutely straight, so (at least in this country) there are always two cables, one above the other.
the top cable is steel (probably steel that does not change much in heat and cold) and it curves from the weight it supports. the bottom cable is copper to carry the electricity, and it hangs from the steel by short wires. those wires get a little shorter in the middle between the towers that hold the steel cable, and are so adjusted 