You write that you would like a more strict value space for crsDefinition. You also write that you would allow any URI. In XML Schema Definition anyURI also allows relative URI references, allowing basically any string, including things like "WGS84", "4326" or "EPSG:4326" (which could in theory resolve in the unlikely case that a resource with that name is available in the same directory as the railML file), or a URN (which requires a dedicated resolver).

Do you have any suggestions on how you would restrict the value space of crsDefinition, syntactically or by best practice?