Simulation of the LPG release, dispersion, and explosion in the Viareggio railway accident Sara Brambilla, Roberto Totaro, Davide Manca Dipartimento di Chimica, Materiali e Ingegneria Chimica “G. Natta” Politecnico di Milano – P.zza Leonardo da Vinci, 32 MILANO, ITALY The manuscript analyzes the accident that occurred in the railway station of Viareggio (Italy) on June 2009. A freight train carrying LPG went off the rails and five out of fourteen wagons derailed and overturned. A hole formed in the first tank car due to the impact with a signaling stake. The pressurized LPG was released as a two-phase jet: the liquid phase formed a boiling pool on the ballast while the dense gas dispersed in the atmosphere. The dense cloud spread and moved towards the neighboring houses. Afterwards, the cloud was ignited and exploded. The overpressure destroyed some residential buildings in the area closest to the explosion epicenter, while glasses were shattered in a larger area. Thirty-one people died and a number of residents were injured due to the fires that engulfed the surrounding houses. Starting from the data related to the tank cars, meteorological data, pictures of the site after the accident, aerial photos, witnesses of both civilians, and rescue teams, the manuscript reconstructs the dynamics of the accident by means of a dedicated software. 1. Qualitative description of the accident On June 29 th , 2009 at 11:48 pm a train loaded with LPG went off the rails while it was crossing the station of Viareggio (Italy). A tank wagon was damaged and the released LPG spread around, finally exploded, and burnt out. Thirty-one people died and more than thirty people were seriously injured. The train transported 14 cars with a nominal capacity of 110 m 3 , each loaded with 45 t of LPG. When the train came in the Viareggio station, the front axle of the first wagon broke and the wagon went off the rails. The first car detached from the tractor, overturned, and dragged nine more cars off the rails. Of the nine derailed wagons, only the first four overturned. The first wagon, which derailed and overturned, was dragged on the ballast, and crashed into a stake that was embedded in the ground. The impact of the tank with the stake produced a longitudinal crack in the metal vessel about 40-50 cm long and a few centimeters wide. When the axle failed, it plowed the ground, sheared several sleepers, and raised a heap of pebbles on the ballast. The drivers felt a strong jerk on the traction; they went to the window and saw the first tank car gone off the rails. The drivers applied immediately the emergency brakes and they could smell the gas. They had enough time to collect the carriage sheets, jump off the train, and run away trampling on the LPG pools on the ballast. Eventually, the drivers took shelter behind a party wall of