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

The European Rail Traffic Management System (ERTMS), as well as its accompanying European Train Control System (ETCS), have promised to interconnect Europe’s railways for over a decade. Yet, for a variety of reasons, this dream has yet to come true. As of October 2019, only 5,700km of track had been equipped with ERTMS, falling short of a target of 7,700km which was set to be achieved by the end of 2019. Delays in the implementation of ERTMS have meant a patchwork of different train control systems exists across the continent’s railway networks, causing difficulties for intra-continental passenger and freight services. 

There is, however, a solution to these woes. Specific Transmission Modules (STMs) can be installed onboard to enable communication between the ETCS-equipped locomotive and the legacy Automatic Train Protection systems. 

What are the advantages of using STMs, especially in the present European context where the implementation of ERTMS faces medium-to-long-term delays? Equally, what challenges do STMs pose, especially regarding their development, testing and certification? 

STMs provide the interoperability promised by ERTMS, but account for the economic and practical realities of implementing what amounts to a major change in how national rail networks function. Some countries are anticipating a wide timeframe in which they will implement ERTMS across their domestic networks. For instance, the government of Norway has set a completion date for ERTMS upgrade in 2030. Therefore, solutions need to be found which enable ETCS-fitted trains to travel along non-ERTMS lines. 

STMs are designed to suit a specific Class B national train control system, translating information received from lineside equipment and balises installed on the track into information which is compatible with the ETCS equipment onboard the train. The STM activates to operate in National Train Control (NTC) level when going from an ETCS-enabled section of track to a non-ETCS section, ensuring signalling commands and other points of interest can be safely transmitted from the control centre via lineside equipment/balises to the driver’s cabin.  

While much of the rolling stock being purchased by NSB (Norway’s national rail operator) will be fitted with ETCS equipment, some will be dual-fitted with ETCS and STMs so as to operate on both ERTMS and non-ERTMS lines. Similar has been done in Denmark, whose ERTMS scheme is due to complete by 2023, where in 2017 Danish freight operators sourced locomotives fitted with dual ETCS-STM onboard equipment.  STMs are thus proving to be an effective stopgap for national rail operators, both passenger and freight-based, before the completion of ERTMS installation across national networks. 

STMs may offer a gateway to interoperability, yet they also offer convenience. Many national networks will possess lines which do not require upgrading to ERTMS, whether this is because they are non-high speed lines or simply because there isn’t enough money to fund such an upgrade. 

In these instances, STMs ensure that rolling stock equipped with ETCS capabilities is still able to travel across a line with several different control systems in operation, using an STM antenna which detects changes in the control system in use and activates when passing onto a track section using a Class B train control system. This includes high speed lines like the Eurostar route from the UK to France, Belgium and the Netherlands, on which a total of seven different control systems are in operation. It is therefore essential that rolling stock is able to transition from one control system to another with ease. 

Implementing ERTMS can be costly, as many European nations are finding out. Multiple components of ERTMS require safety level integrity 4 certification (SIL4), leading to increased expense when equipping rolling stock and track to be ETCS-compatible. 

STMs are one such component which must be SIL4 certified, requiring extensive testing and documentation coverage across the entire system. This naturally means that extensive testing and certification activities are required in order to implement an STM on a rail network. This is compounded by the fact that individual STMs need to be developed and implemented for each individual national rail network. For instance, in a project where STMs were developed for the networks of Sweden, Norway and Finland, a total of 1300 test scenarios were executed.  

One solution to this challenge is test automation. Automated STM testing usually needs to be bespoke due to the various interfaces with which they interact onboard the train. System tests build a virtual environment accounting for rolling stock type, driver actions, whether or not ERTMS is implemented on a certain line, amongst other variables which could affect the STM. 

While automated testing frees up engineers’ time so that they can work on other integral tasks relating to STM development, it requires significant expertise in order to do well. Rail manufacturers therefore need to bear this in mind when developing STMs. 

Yet while they may be expensive to test and certify, they do provide simplicity to drivers when interacting with different train control systems between networks. Different STMs are used for different national networks, yet this shouldn’t prove a challenge to drivers as long the train is equipped with the necessary modules. 

A key reason behind equipping rolling stock with STMs is to simplify information flow from the train to the driver, ensuring that this information is displayed in a clear way so as not to cause potentially serious driver errors. The intelligible user experience provided by STMs ensures that drivers do not ‘lose control’ of the system so to speak, avoiding any errors when it comes to misinterpreting upcoming signals and speed warnings. 

STMs provide a wealth of opportunity at a time when ERTMS implementation is at different stages on different rail networks across Europe. While they come with their own challenges, including the extensive testing and SIL4 certification they require, there is no doubt that they provide a useful stopgap while more and more kilometres of ERTMS are rolled out across the continent. 

For more on Critical Software’s work in testing and certifying systems to SIL4 requirements, as well as our other testing and certification expertise, take a look below.