I know a power transmission line carries usually two sets of 3 phase conductors. But I have noticed a relatively thin wire at the center-top of the power transmission line. What is the purpose of this thin wire?
Wikipedia says they are ground wires to protect the live wires from lightning.
Top of the cable is ground wire rest and other (1 or 3 lines) are phase cables. It is just for the protection from lighting strikes. They serve to shield the line and intercept lighting stroke before it hits the current carrying conductors below. That's why they are often made of steel.
As stated when they use the ground wire for communication it have the signal inside its. And yes, the ground wire is grounded on each tower (one of the best transmission type for protection of electric systems is the fiber optical inside a grounded wire). In some arrangements, there are more than one guard cable. It can be up to 3, depending mostly of the size of tower and placement of the phase cables. The worst case I've ever seen is below