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Within the interdisciplinary research project METRO, two full-scale fire tests were performed with ignition inside commuter train carriages in a tunnel. Both tests developed to fully flashover conditions. The fire development was very different in the two tests. The main reason was the difference in initial combustion behaviour between the case with combustible wall and ceiling lining, and the case with a refurbished carriage using aluminium sheet covering the combustible lining as the exposed interior surface. In the case with combustible lining a ceiling flame was developed, radiating towards the seats and the luggage spreading the fire more quickly than in the case without exposed combustible lining. Also in the gas concentrations, significant differences could be observed between the two tests. During the tests, concentrations of O₂, CO and CO₂ were sampled and analysed at three different heights. The paper focuses on the time resolved results of the gas concentration. The development in gas concentration at different levels is presented and discussed in relation to the fire development in the carriage. Results from calculations of time to incapacitation and fractions of an incapacitating does are also included.