The M4 project or "Blue Line", which sees Webuild as agent of the awarded ATI, involves building a fully automated, driverless metro to connect Linate airport with Milan's San Cristoforo railway station.

The new line, built entirely underground, for a length of approximately 15 km, from Linate to Lorenteggio, will provide a fast connection, along the east / south-west route, through the historic city center.

The contract involves the design (definitive and executive) and construction of two single-track tunnels, one in each direction, with 21 stations, 30 associated structures and a depot/workshop.

The new line will mainly be developed in the Municipality of Milan, encroaching, near the eastern terminus, for a small portion into the area of the municipalities of Segrate and Peschiera Borromeo. There are plans for two intersections with the existing metro lines – with the red line near the station of San Babila and with the green line near the station of Sant’Ambrogio – three intersections with the suburban railway lines – at the Forlanini FS station, the Dateo station and the San Cristoforo station – and one intersection with Linate airport. These intersections will increase the intermodal “network effect” expected from the entire urban and non-urban transport system.

It involves a light and fully automated driverless metro system, with automatic platform screen doors and a CBTC signalling system (Communication-Based Train Control); the fleet will comprise 47 vehicles with 4 intercommunicating carriages.