An intense debate has been taking place recently about the costs and profitability of high-speed lines. This article aims to help document one of the most widely argued aspects of this debate and, in our opinion, one that has been addressed most hastily: the costs included in the income statement of infrastructure operations, especially those relating to asset depreciation. The article therefore reviews, firstly, those assets included on the infrastructure manager’s balance sheet and the evaluation criteria applied to them. It also takes the chance to take a comparative look at unit investment costs internationally, to point out the elements that determine the level of these costs and to analyze a key feature of these investments, their indivisibility.