But when the tunnel was finally built in 1930, it was taken over by the Underground system as part of Line B, and as a result, suburban passengers had to change at Federico Lacroze, named after its builder, about 6 km (3.7 mi) from the centre.

[5] Three years later, in 1891, it was converted to steam and as the capital expanded, business to the suburbs was so good that a new branch to Campo de Mayo was inaugurated in 1904 using electric power supply, and the whole section between Federico Lacroze and San Martín was electrified in 1908.

Electricity came from the Lacroze tramways of Buenos Aires and overhead wires delivered 600 volt DC current to a fleet of wooden US-style interurban coaches from Brill.

The original cars were still running by the end of the 1940s and had become too unsafe to permit continued private operation, and the line was nationalised by Juan Domingo Perón's government in 1948 and became part of the General Urquiza Railway, one of the several divisions of the state-owned Ferrocarriles Argentinos.

By 1951 it was completely rebuilt, new substations were set up, new modern cantilever roofed stations made of reinforced concrete were built and 28 used 1700 series interurban coaches, manufactured by Pacific Electric Railway between 1925 and 1928, were bought.