The order, which will largely be financed through loans from Chinese banks, was announced during a visit to CNR facilities in China by Argentina's transport minister Mr Florencio Randazzo in early October.
The trains will be delivered between October 2014 and February 2015 and will operate in multiple during peak periods.
A further order for 32 three-car dmus for the non-electrified sections of the 1676mm Mitre and Sarmiento networks will be placed in the near future and tenders have been sought from Chinese suppliers.
Both CNR and CSR have won major contracts in Argentina in recent years as the government plans to renew almost all commuter rail equipment over a five year period. Both Chinese companies are planning to establish assembly facilites in Argentina.
Operation of the Belgrano Sur service had been the responsibility of the UGOFE private consortium of train operators, which replaced the previous private concessionaire Metropolitano in 2007. In August the government announced that state-owned operator SOFSE would take over operation of the line within three months.