Where , before an authorisation is issued by the national safety authority , the Agency becomes aware or is informed by the applicant through the one - stop shop in accordance with Article 19(6 ) of Directive ( EU ) 2016/797 that a project design or specification was changed after the Agency had issued an approval in accordance with Article 19 of Directive ( EU ) 2016/797 and that there is a risk of a lack of technical and operational compatibility between the ERTMS trackside subsystem and vehicles fitted with ERTMS , it shall cooperate with the parties involved , including the applicant and the relevant national safety authority , in order to find a mutually acceptable solution .