This study presents the chemical characterization and the source apportionment of particulate matter (PM10) in an underground railway station. The sampling site is the platform of an underground station operated by SNCF in France. Outdoor PM were also sampled on the roof of this station. During the 5 measurement campaigns conducted for 8 months, the mean concentration of PM10 was 129.78 µg/m3 in this underground station. PM10 sampled on the platform was composed of 52% metals and elemental compounds, mostly iron, silicon, and barium. Elemental carbon and organic matter represent respectively 8% and 14% of the mass of PM10. Source apportionment using Positive Matrix Factorization (PMF) analysis shows that in this underground station, 82.1% of PM10 is emitted by railway traffic. 17.9% of the PM10 comes from the outdoor air. However, railway sources have similar chemical compositions, and emissions from these sources occur simultaneously. Consequently, this study reveals the difficulty of further separating emissions from railway traffic using the PMF model. It underlines the need to expand our knowledge of the chemical composition of railway emissions and to define more specific tracer compounds of each railway source of PM.