TSI — Technical Specification for Interoperability

A TSI is binding EU law — adopted as a Commission regulation under the Interoperability Directive (EU) 2016/797 — that translates the directive's essential requirements into concrete technical requirements for one subsystem of the rail system: infrastructure, energy, rolling stock, control-command and signalling, operation, telematics, noise, accessibility, or tunnel safety. Unlike an EN standard, a TSI is not optional for its scope: placing a subsystem in service in the EU requires an EC verification procedure in which a Notified Body assesses conformity with the applicable TSIs. TSIs cite EN standards and other specifications as means of compliance — sometimes making a specific document mandatory, as the CCS TSI's Annex A does for the ETCS subsets. Where a TSI intentionally leaves a topic unregulated (an 'open point') or grants a member state a deviation (a 'specific case'), notified national rules fill the gap. The nine TSIs were substantially revised as a package in 2023, and ERA publishes application guides explaining their shared mechanics.