This tutorial is aimed at demonstrating the capabilities of computational fluid dynamics (CFD) models in capturing complex dynamic scenarios. In particular, the focus is on the dispersion of flammable heavy gas in urban areas. The necessary steps for carrying out a dispersion study on a large scale are described. The methodology is applied for the analysis of a case study: the accident in Viareggio, Italy, in 2009, in which a flash fire occurred after the derailment of a freight train. It shows how CFD models allow effective reproduction of large-scale release and dispersion events in the presence of a congested and complex environment. In parallel, limitations of conventional approaches are also shown for the same case study.