RailFans - we wander, but never get lost - Shaurya Kumar

This is probably a 'return conductor'.

Basically, every electrical circuit needs to be a closed loop (Like how a battery has two terminals and every thing has to be connected to both terminals of the battery with two wires, to work).

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explain in simple terms (though this explanation is technically NOT very accurate), think of the generator at the power station as a having a positive and a negative terminal like a battery. To make the motor in an electric loco work, the positive and negative terminals of the motor have to be connected to the positive and negative terminals of the generator. (There is no positive or negative terminal per se, for an AC alternator, this is only for the sake of explanation)

But we see that our OHE normally has only 1 wire which is in contact with the pantograph (Think of this as the wire connecting the positive terminals). So a second connection is required between the negative terminals to complete the circuit.

Normally, the rails themselves are used for this. Current coming from the OHE passes through the traction motors, and then through the wheels of the loco into the rails below. At regular intervals, the the rails are grounded. Similarly one end of the generator is also grounded. So the earth itself acts as the other wire.

However, the rails are also used for track circuits to control signalling, and there will be railway/telecom communcation cables also near the railway tracks. In some cases, these cables will be close enough to the tracks that the return current in the steel rails can affect the signals in these cables. In these cases, a separate 'return conductor' is provided (which is basically just a wire which is connected to the rails at regular intervals) high above the ground, to minimize the interference.