Railway electrification from Chhatrapati Shivaji Maharaj Terminus (the then Victoria Terminus) to Kalyan was commissioned on 12.11.1928 with 1500-volt DC traction system. Electrification was subsequently extended up to Igatpuri in Northeast line and up to Pune in Southeast line. First passenger train Deccan Queen, with electric loco WCP-1 ran on 1.6.1930 between Bombay VT (now CSMT) and Poona (now Pune).
In 1500-volt DC system over head equipment power was being supplied at 1500 volt from rectifier fed traction substations (TSS) normally having no sectioning. i.e. the TSS were connected in parallel. Traction Substations inter spacing being very low, making fault level unmanageable and other problems. Over head DC traction power supply system had reached its saturation level and it was very difficult to run more trains in section. Recurring maintenance cost of DC traction system was also very high. High number of faults also occurred due to very high currents in the DC system.
The 25 kV AC system emerged as an economical form of electrification as a result of research and trials in Europe, particularly, on French Railways (SNCF). Indian Railways decided to adopt the 25 kV AC system of electrification as a standard in 1957.
Considering the limitations of the existing DC traction system, a decision was taken to convert it to 25 kV AC traction in 1996-97. Conversion from DC to AC traction of Igatpuri-Kalyan section was completed on 12.11.2006; Pune-Lonavala-Kalyan section on 24.10.2010; Kalyan-Thane section on 24.1.2014 and Thane-CSMT on 8.6.2015 on Central Railway. With this, the entire electrified mainline rail network in Central Railway uses 25 kV AC.
Due to switching over from 1500 v DC traction to 25 kV AC traction, number of Traction substations have reduced from 21 to 4 in Kalyan-CSMT and from 72 to 18 in CSMT-Igatpuri/Pune section.
There is a saving of approximately Rs.120 crore per annum in electricity bill due to conversion from DC to AC as transmission losses have reduced, regenerative braking in three phase EMUs and locomotives has been introduced etc.