Full STM Ahead! The Promise of Specific Transmission Modules on European Rail Networks

The European Rail Traffic Management System (ERTMS) and its accompanying European Train Control System (ETCS) have promised to interconnect Europe’s railways for over a decade. Yet, for various reasons, this goal has not been fully achieved. As of October 2019, only 5,700 km of track were equipped with ERTMS, falling short of the 7,700 km target set for the end of that year. These delays have resulted in a patchwork of train control systems across national networks, complicating passenger and freight operations.

A potential solution is the use of Specific Transmission Modules (STMs) onboard trains. STMs enable communication between ETCS-equipped locomotives and legacy Automatic Train Protection systems.

STMs provide the interoperability ERTMS promised while accounting for the economic and practical realities of implementing large-scale network changes. Many countries have extended timelines for ERTMS implementation. For example, Norway aims to complete its ERTMS upgrade by 2030. In such cases, STMs allow ETCS-equipped trains to operate on non-ERTMS lines.

STMs are designed for specific Class B national train control systems. They translate information received from lineside equipment and track balises into data compatible with onboard ETCS equipment. When moving from an ETCS-enabled section to a non-ETCS section, the STM activates in National Train Control (NTC) mode, ensuring signals and other critical information are safely transmitted to the driver.

For example, Norwegian State Railways (NSB) has purchased rolling stock fitted with ETCS, while some trains are dual-fitted with ETCS and STMs to operate across both ERTMS and non-ERTMS lines. Denmark has followed a similar approach, equipping freight locomotives with dual ETCS-STM systems prior to the completion of its ERTMS rollout in 2023.

STMs also offer convenience. Some lines may not require ERTMS upgrades due to their non-high-speed nature or limited funding. In such cases, STMs allow ETCS-equipped trains to operate across multiple control systems seamlessly.

High-speed lines such as the Eurostar route from the UK to France, Belgium, and the Netherlands use seven different control systems. STMs ensure rolling stock can transition smoothly between these systems, avoiding operational errors.

ERTMS implementation is costly. Many components, including STMs, require Safety Integrity Level 4 (SIL4) certification, which demands extensive testing and documentation. Each national rail network requires tailored STM development. For example, developing STMs for Sweden, Norway, and Finland involved executing 1,300 test scenarios.

Automated testing can help streamline this process, simulating rolling stock types, driver actions, and line-specific ERTMS configurations. While automation reduces manual workload, it demands high expertise from rail engineers.

Although STMs are expensive to test and certify, they simplify operations for train drivers. Drivers interact with different national control systems seamlessly, as STMs provide clear, intelligible interfaces. This reduces the risk of misinterpreting signals or speed warnings, enhancing safety.

STMs offer a practical solution while ERTMS implementation progresses at varying rates across Europe. Despite challenges like extensive testing and SIL4 certification, STMs serve as an essential stopgap, enabling safe and interoperable train operations across national networks.