The FL4 (until 2012 FR4) is a regional rail route. It forms part of the network of the Lazio regional railways (Italian: ferrovie regionali del Lazio), which is operated by Trenitalia, and converges on the city of Rome, Italy. The route operates over the infrastructure of the Rome–Cassino–Naples, , Rome–Frascati and railways. Within the territory of the comune of Rome, it plays the role of a commuter railway. It is estimated that on average about 55,000 passengers travel on an FL4 train each day.