The fire dynamics of a train compartment was investigated in detail. A full scale test was performed which showed that the compartment reached flashover three minutes after ignition. Small scale tests were performed on the wall and floor linings and on seat and table materials. The results from these tests were used as input to a CFD-model implemented in FDS5 (Fire Dynamics Simulator, v5). It was possible to obtain a good correlation between the FDS5 model and the full scale experiment. However, in order to obtain this correlation it was necessary to deviate significantly from the small scale experimental results regarding the thermal conductivity of the seat material. Further, the FDS5 model was highly sensitive to grid size. Despite these drawbacks it was concluded that FDS5 can be used to determine design fires for the tested compartment and for geometries and material selections which do not differ drastically from the tested configuration. A much simpler way to obtain a design fire is to use so called t2 pre-flashover fires. Using this approach it was found that the growth rate of the compartment fire was between fast and ultra fast, with characteristic time scales of 150 s and 75 s, respectively. Finally it was concluded that the main part of the heat release originated from the seats and that a reasonable good design fire can be found by studying the fire dynamics of individual seats only.