The completion of the Pesos 297m ($US 7.7m) project takes the total extent of electrification on the Roca Line to 71km.
With the completion of electrification, new EMUs supplied by CRRC Qingdao Sifang have been introduced on services between Berazategui and Bosques.
According to the Ministry of Transport the line is used by around 21,000 passengers a day.
The 237km Roca Line network encompasses eight branches with 69 stations and carries around 550,000 passengers a day.
Last October electrification was completed on the 9.6km branch from City Bell to the state capital La Plata.