This article develops an econometric analysis of metro station operating cost to identify factors that create variation in cost efficiency. Stations operating costs can be classified amongst the semifixed costs that a metro faces in the sense that they do not vary proportionately with metro output. They may therefore be important in determining the degree of returns to density. This article seeks to provide an improved understanding of some of the major factors driving these costs. Empirical results show that strong system-specific influences impact costs but over and above these we detect positive associations from a range of station characteristics, including the length of passageways, number of platforms, peak-level service frequency, interchange demand, and the provision of toilet facilities. In addition, we find that the presence of air-conditioning has a substantial effect in increasing expected station operating cost by as much as 40 percent.