In order to facilitate, improve and develop international rail transport services and the progressive creation of the internal market in equipment and services for the construction, renewal, upgrading and operation of the rail system, the principle of mutual recognition of the national rules, the checks against these rules and the associated authorisations to place into service is supported by the Cross-Acceptance Unit works.National Reference Documents:The Cross-Acceptance Unit also publishes National Reference Documents for the authorisation for placing in service of railway vehicles. These can be found in the Technical Evaluation Sector section and since 1st of APRIL 2012 in the Reference Document Database (RDD).