A Technical Specification for Interoperability (abbreviated as TSI) is a text provided for in European Directive 2016/797 adopted by the European Parliament and the Council of the European Union on the interoperability of the European rail system in accordance with the ordinary legislative procedure.

This directive stipulates that the railway system is divided into 8 subsystems:

It also provides that a technical specification for interoperability (TSI) is drawn up for each subsystem.

These TSIs define the essential requirements of the above-mentioned European directives for specific cases and define a set of technical requirements that apply to new subsystems put into service.

These requirements constitute a set of conditions necessary for putting into service, but these conditions are generally not sufficient to guarantee safety, so they must be supplemented by some additional measures. They do not cover all the fields of the regulatory requirements, but for the fields they cover, they prevail over the national texts.

When the first TSIs were published, they still had separate legal bases: one for the interoperability of the high-speed rail system (European Community Directive 96/48/EC) and one for the interoperability of the conventional rail system (European Community Directive 2001/16/EC).

The matters covered by these two directives have been merged and regrouped in the European Community Directive 2008/57/EC, which in turn has been taken over by the European Union Directive 2016/797/EU.

Directive 2008/57/EC was repealed as of 16 June 2020, by which time the new Directive 2016/797/EU had to be transposed into national law.