Three fire tests wereperformed under and inside commuter train carriages in a tunnel. Both tests initiatedinside the carriage developed to fully flashover conditions. The time toflashover was significantly different between the two cases. In the test with theoriginal seats and linings the maximum heat release rate (HRR) was 76.7 MW and occurred12.7 min after ignition. The maximum HRR in the case where more modern seatsand aluminium lining were used, occurred after 117.9 min. The main reason forthe difference was the difference in initial combustion behaviour between thecase with combustible wall and ceiling lining, and the case with aluminium as theexposed interior surface. In the case with combustible lining a ceiling flame wasdeveloped, radiating towards the seats and the luggage spreading the fire morequickly than in the case without exposed combustible lining. When the growthrate of the fire was rapidly increasing, a flame was observed at the ceiling.The maximum HRR calculated from the experimental results are significantlyhigher than those obtained in other documented test series. The luggage in,under or between different seats is presumed to increase the fire spreadsignificantly in both cases. This was obvious from results performed within thesame project prior to the full-scale tests.