EULYNX is developing the specifications for standardised technical interfaces for railway signalling systems across multiple European railway infrastructures. We have successfully established an assurance approach, based on CSM-RA, to assure the interface specifications. Using CSM-RA aligns EULYNX to the acceptance processes within the countries where the interfaces will be used. Assurance of EULYNX aims to give confidence to product developers that products using these interfaces will be acceptable on railways across Europe. We have completed an initial baseline of the assurance and are now extending this to consider further details of the EULYNX interfaces