The Diabolo project is creating a new railway line serving Brussels National Airport.
It is so called because of its shape: the line forms a chord between the Brussels-Leuven line going east and the Brussels-Antwerp line going north, and connects with them via a triangular junction at each end. Its building will permit a much more flexible airport railway service to be laid on: as well as mainline trains going north and east, future high-speed services between London, Paris, Amsterdam and Cologne could call there. This would make Brussels National Airport a fully-fledged international railway station on the model of Amsterdam's Schiphol or Paris's Charles de Gaulle. At present, the airport station is a terminus, linked only to the Brussels-Leuven line.
As of May 2009, work is well advanced on the new line (numbered 25N) being built up the central reservation of the A1/E19 motorway from Schaerbeek/Schaarbeek to Mechelen: the bridge span connecting it to the existing line at the northern end was placed in May 2009 and the tunnel into the airport at the southern end is being dug. Plans to open the line by March 2012 have been pushed forward, after some fears were shown as to the completion date of the project. This opening will then let to the beginning of another important Brussels-based rail project in terms of infrastructure construction: the Brussels RER, whose inauguration is planned between 2015 and 2018.