In India, Voltages used for Electric Traction is 25kV AC. In some regions 1.5kV DC is also being used. Rest of the India is using 25kV AC.

In case of 1.5kV DC, Positive terminal is connected to Catenary and negative terminal is earthed through railway track.

If we talk about 25kV AC which is being widely used across the world for high speed trains, runs on overhead line & pentagraph is used to give supply to the train. The phase is connected to overhead line via Pentagraph & neutral is earthed through railway tracks for completing the loop. 25kV AC is stepped down with the help of a Traction Transformer thereby converting the AC to DC through rectifiers. DC is mostly preferred as DC motors have advantage of easy speed control.

As only one cable/wire is visible doesn’t mean Electric Traction works with Single Wire. Its a railway track which act as a 2nd wire for completing the loop/circuit.