What is the purpose of the 'Service Line' (Blue) in a combination vehicle?

Question

Accepted Answer

It carries air, which is controlled by the foot brake or trailer hand brake.. Section 6.2: The service line (also called the control line or signal line) carries air which is controlled by the foot brake or the trailer hand brake.

Answer

It carries air to the trailer air tanks.

Answer

It provides electricity.

Answer

It is for emergency braking.