Specific Transmission Module

Critical Software's Specific Transmission Module is an onboard train protection system that connects Portugal's legacy CONVEL train control system with the European Train Control System.

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What the STM does

The STM enables railway interoperability without infrastructure changes, letting operators run ETCS-equipped trains safely across CONVEL-signalled lines today, while Portugal completes its ERTMS legacy migration.

First of its kind in Portugal, the STM is SIL 4-certified and ERA TSI-compliant: a turn-key, vendor-agnostic solution built for a smooth transition into European railway interoperability.

The STM is an add-on onboard unit that allows ETCS-equipped trains to read and respond to legacy CONVEL balise signals in real time. Without it, and due to an obsolescence constraint, no new trains can start operating in Portugal, nor can existing fleets be upgraded to the ETCS protection system.

Installing the STM removes that constraint. Operators avoid costly parallel fleets or premature infrastructure overhauls. New CP regional trains and Medway freight locomotives can run anywhere in the country from day one. Foreign or leased trains with ETCS can enter Portugal without extensive modifications.

The interoperability works in both directions: as infrastructure transitions to ETCS, existing CONVEL-dependent trains won't be stranded.

Key Differentiators

Six reasons the STM stands apart, from full CONVEL rule coverage and SIL 4 certification to a vendor-agnostic design built to outlast a 20-year migration.

Train control system interoperability

Critical Software's STM implements 100% of the legacy CONVEL rules within the ETCS onboard environment, keeping all vehicle command and control with the ETCS system, as defined by the applicable TSI and subset requirements. Trackside equipment stays unchanged.

SIL 4 safety certification

Engineered to Safety Integrity Level 4, the highest level for railway control systems. Developed in accordance with EN 50126 / 50128 / 50129 and tested in both laboratory and real-world conditions. An independent European Notified Body (NoBo), a Designated Body (DeBo), and an Independent Safety Assessor (ISA) are involved in certifying the STM as a Generic Product. Full SIL 4 certification is on track for 2026.

ERA TSI-compliant

Fully compliant with the European Railway Agency's Technical Specifications for Interoperability, ensuring the STM meets the standards required to operate across the European rail network.

Secure by design, ready for the CRA

The STM's development lifecycle incorporates secure-by-design principles, building on Critical Software's established SIL 4 safety and quality processes under EN 50128 and EN 50129, complemented by cybersecurity practices aligned with IEC 62443 and CENELEC TS 50701 for railway cybersecurity.

Vendor-agnostic, modular design

Compatible with different ETCS onboard equipment suppliers and versions. Future ETCS software upgrades won't break STM functionality. Operators can adopt new ETCS technologies or suppliers without losing CONVEL compatibility, supporting an open market for signalling in Portugal.

Built for the long haul

Industry estimates place full ETCS implementation across Portugal's track network more than 20 years away. The STM is designed to remain a relevant, supported solution for the entire duration of that transition, giving fleet operators and lessors a stable, long-term integration path.

Development Timeline

From kick-off in 2022 to certification and rollout in 2026, the STM has progressed through vehicle installation, on-track testing, and public demonstration. Here's how the programme has advanced toward full SIL 4 certification.

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2022

Project development kick-off

2022

2023

First vehicle installation

2024

Initial tests on track

2025

Public demonstration

Q1 2026

Final dynamic tests

Q3 2026

Certification and rollout

2026

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From Portugal to Europe

Portugal's CONVEL-to-ETCS challenge is not unique. Many countries operate Class B ATP systems alongside ETCS as they migrate to the ERTMS standard. The STM's generic certification and turn-key architecture mean the solution can be adapted for other national legacy systems, positioning Critical Software at the forefront of a common European rail interoperability problem.

Whether you operate passenger or freight rolling stock, manage a leased fleet, or are evaluating ETCS migration options, our team can outline integration scope, certification status, and delivery timelines for your specific fleet. Fill in the form and we'll get straight back to you.