Rail interoperability in Europe explained
European rail interoperability is the legal and technical framework that determines whether a train authorised in one country can operate across borders — and why, in most cases, it still cannot without significant extra cost and time.
The European Union began legislating for rail interoperability in 1996, when Directive 96/48 introduced common technical requirements for high-speed trains. The groundwork had been laid five years earlier by Directive 91/440, which required member states to separate infrastructure management from train operations and open the network to access. The goal was a single European railway area where rolling stock, signalling and infrastructure would meet common standards.
The operative instrument is a set of Technical Specifications for Interoperability (TSIs) — binding EU regulations that define what each subsystem of the rail network must do to be considered interoperable. TSIs cover infrastructure, energy supply, rolling stock, control-command and signalling, and operations.
What TSIs do — and what they do not
A TSI sets the technical floor for a given subsystem. A piece of rolling stock that meets the relevant TSIs can be placed on the EU market without being re-engineered for each country it enters. A stretch of track built to TSI standards can, in principle, receive any TSI-compliant train.
In practice, TSIs do not override every national requirement. Each TSI contains specific cases — permanent or temporary derogations for particular national conditions — and open points where no EU-level standard yet exists and national rules still apply. A vehicle authorised under EU rules may still require additional checks before operating on a specific national network.
TSIs are drafted by the European Union Agency for Railways (ERA) on a mandate from the European Commission, in cooperation with national safety authorities and industry representatives. They are revised periodically, most recently as part of a digital rail and green freight package initiated in 2020.
ERA’s role: from standard-setter to authorisation body
ERA, based in Valenciennes, France, has held a growing institutional role since it was established in 2004. Under the Fourth Railway Package — the legislative framework that took full effect from 2020 — ERA became the primary authorisation authority for rolling stock intended for cross-border use across the EU.
Before 2020, a manufacturer seeking to operate a new train type across several member states had to apply separately to each national safety authority. ERA’s one-stop shop replaced that process for multi-member-state authorisations: a single application, assessed by ERA in coordination with the relevant national authorities, produces one authorisation valid across the member states specified in the application. Purely national vehicle authorisations continue to be handled by national safety authorities.
ERA also maintains the key registers underpinning the system — including the European register of authorised vehicle types and the register of infrastructure, which operators use to check route compatibility before deploying rolling stock on a new corridor.
The barriers that remain
Technical harmonisation has advanced, but the physical differences between national networks pre-date the EU and cannot be legislated away quickly. Three categories of barrier define where the system still breaks down.
Track gauge is the most visible. Most of Europe operates on 1,435 mm standard gauge, but Spain and Portugal use 1,668 mm and the Baltic states inherited the 1,520 mm gauge from the Soviet-era network. A train built for standard gauge cannot operate on broad-gauge track without a gauge-changing axle or a physical transfer.
Electrification is a second barrier. At least four voltage systems are in operation across the EU: 25 kV AC, 15 kV AC, 3 kV DC and 1.5 kV DC. Multi-system locomotives and trainsets can handle several standards, but they are more complex and more expensive to build and maintain than single-system vehicles.
Signalling was the original driver of the interoperability project. By the early 1990s, up to fourteen incompatible national train protection systems were in use across Europe. The European Train Control System (ETCS) — the signalling component of the broader ERTMS framework — is replacing them incrementally, but migration is slow and legacy systems remain in operation on most national networks.
A fourth layer of friction sits in national rules. ERA has led a clean-up programme that, according to ERA, reduced the number of national technical rules relevant to vehicle authorisation from around 14,000 to approximately 1,000. The process is not complete, and the remaining rules continue to add time and cost to cross-border authorisations.
Meeting those conditions depends on infrastructure investment, agreed migration schedules and decisions on legacy system retirement across member states.
Related:
Why running trains across European borders is so hard
EXPLAINER: European trains are harder to run across borders than within them — and the reasons are technical, institutional and political, not geographic.