There is also a third line that goes from the tractor to the trailer; the trailer electrical connector cable that powers the lights.
It can be seen below as the heavy black cable that is coiled up on the back of the truck. It connects on the truck deck around the same area as the air hoses. It goes to the connector on the front of the trailer.
Additionally, this pic shows that the air lines can be either blue or yellow for the main service line, and are usually red for the emergency line.