National technical rules (NTRs) supplement National Technical Specification Notices (NTSN) with network or country-specific requirements.
NTRs can perform several functions, such as:
Under Regulation 3C of the Railways Interoperability Regulations 2011, the Secretary of State for Transport must publish a list of all national technical rules (NTRs) applying in the United Kingdom and the list must, where appropriate, specify the notified national technical rule (NNTR) that an NTR replaces.
These are the lists of NTRs for infrastructure and vehicles that replace the lists of NNTRs previously notified to the Commission by the Secretary of State.
https://www.gov.uk/government/publications/rail-interoperability-current-national-technical-rules