Abstract: Electric Traction in Mumbai has successfully completed the conversion of overhead power supply system from 1500 Volt D.C. to 25KV A.C. June 2015. This is nothing short of an achievement considering the fact that engineers did it without disturbing current services and creating trouble for the 42 lakh commuters in their day-to-day travelling. Electric Traction is oldest (1925) and largest mode of transportation in Mumbai city, connecting various suburban to commercial center. This paper studied the harmonics due to this A.C.Electric Traction in supply grid, at Thane Traction substation in Mumbai. High reliability and reduced maintenance of AC motor has also enabled Railway boards to increase the number of train. This paper compares the voltage and current patterns and harmonic presence in AC supply at Traction substation.