CPV code 50225000 designates Railway-track maintenance services in the Common Procurement Vocabulary (CPV 2008), the EU classification used to describe the subject of public contracts.
50225000 is a leaf code — the most specific level of its branch — so it can be used directly as the main CPV code of a contract notice. If it does not quite fit, check the neighbouring codes under 50220000 (Repair, maintenance and associated services related to railways and other equipment).