The Porrettana line was immediately successful but while it was a significant engineering achievement it soon became clear that it was inadequate to cater for the traffic attempting to use it.

Various projects for additional crossings of the Apennines were developed, and the Pontremolese line was opened in 1892 between La Spezia and Parma providing an alternative route from Rome to Milan.

In 1882 the designer of Porrettana, Jean Louis Protche, was commissioned by several institutions to examine a number of projects including one dating back to 1871, proposed by the engineer Antonio Zannoni which proposed a more easterly route for the new line in order to reduce the length of climb.

In 1902, the Colombo Commission was established to study the proposals of Protche and Zannoni along with those of several other engineers, Sugliano, De Gaetani, Naldoni and Mercanti.

The project was then approved for a double track line using electric traction, widely spaced stations, 31 tunnels—which alone constitute 37% of the line—and numerous bridges.

Construction lasted from 1913 to 1934, slowed by World War I, economic and political crises and the complexity of the work.

An aerial rope way was also built from the northern entrance to carry construction material to a work site built at Cà di Landino above the centre of the tunnel in the town of Castiglione dei Pepoli; Cà di Landino was connected to work sites in the Apennine Tunnel by two additional tunnels (both over 500 metres long) built at an angle of 27 degree.

On 22 April 1934 the line finally opened to traffic and revolutionised Italy's transport system, just as the Porrettana railway had done 70 years before.

It was intended primarily to allow the overtaking of slow goods trains and had two lines over 450 meters long dug parallel to the main tunnel.