The life-cycle of your relay-based interlocking system has come to an end. Now what? 

For many infrastructure managers, the desire to embrace the digital age with the freedom and possibilities a computer-based interlocking (CBI) system can offer is clear. But making the decision to actually go through with the transition can be less straightforward. After all, it is never a small undertaking to upgrade an interlocking system, let alone switch to a completely new type of system. Luckily, there is a solution that can help ease the transition while giving you more control over the entire system development process—from writing specifications to commissioning and maintenance.

Why upgrade to a computer-based interlocking system?

When it comes time for a system upgrade, it can often feel like the easiest route would be to upgrade the current relay-based interlocking (RBI) system. And many do, for good reasons. RBI systems are simple, reliable, and have been proven in use for a long time. However, they can also hold you back.

Among other limitations, RBI systems have a limited amount of memory which restricts the kind of functionality that can be added to modernize and keep up with changing system needs. Additionally, because relay is an old technique that is becoming less and less common, it can become difficult to access the resources needed to continue running a RBI system in the future—both in terms of spare parts and people who possess the necessary competence and education. There are also challenges related to the system upgrade transition itself. In order to write requirements for your new system, as an infrastructure manager, you must first understand how the current system works and, unfortunately, documentation for older RBI systems is either incomplete or missing.

Pros and cons of RBI systems

ReliableLimited functionality
Proven in use for a long timeDocumentation can be missing or insufficient
SimpleCompetence hard to find or not available
Long life cycle (40 – 60 years)Spare parts not manufactured anymore or hard to come by

CBI systems, on the other hand, can help you break free from the limitations and challenges of RBI systems. Although there are some drawbacks to consider. CBI systems have shorter life cycles compared to RBI systems, 20-25 versus 40-60 years. They can also introduce increased complexity if used incorrectly. However, all in all, the benefits of upgrading to a CBI system far outweigh the drawbacks.  

Instead of mechanical interlocking relays, modern CBI systems utilize interlocking computer software. This requires a shift in thinking. Since you are developing a computer program, you don’t have to restrict yourself to mimicking the setup of the old relay system. A CBI system offers greater capacity and freedom to build the system and functionality you want. Documentation and competence are more readily available. CBI systems can also be verified for meeting safety requirements using modern tools.  

Pros and cons of CBI systems

Modern – interlocking computer software instead of mechanical interlocking relaysIncreased complexity if used incorrectly
Increased functionalityShorter life cycle (20 – 25 years)
Can be verified with modern tools
Documentation and competence available
Increased capacity

Easing the transition using a digital twin

If you have decided to go ahead and upgrade to a CBI system, the challenge remains of how to make the transition as smoothly as possible without interfering with your existing system or disrupting traffic. The solution? Creating a digital twin. Encompassing your entire infrastructure—from stations, rolling stock, and signals, to the coordinating IT systems—a digital twin is a virtual, interactive replica of your physical system, asset or process, including its real-time characteristics and behaviors.

When it comes to determining how successful the transition to your new CBI system will be, writing clear specifications for the development of your new system is a critical first step. Any errors or omissions in your specifications will have a negative impact on the later steps in the tender and development process and, ultimately, determine whether or not you get the system you want. A digital twin will help you gather the input you need to write more accurate specifications from the start, and gain greater control over your system, from development and delivery to ongoing maintenance.

Gain system control and unlock new possibilities

Having a digital twin model of your current interlocking system will enable you to get the input you need to understand how your current system behaves, and specify requirements for the new system. You may decide to program the future system to act exactly the same, or perhaps you will decide to make improvements and remove any oddities present. You may even decide to add brand new functionalities. 

Prior to deciding on system specifications, your digital twin allows you to play around and test different functionalities and scenarios without interfering with the real physical system. If you are lacking any documentation, you can use the digital twin as a tool to conduct reverse engineering and fill in any information blanks. Once you are clear about how you want your new system to work, you can extract clear system requirements from the digital twin in the form of an object model describing all objects and their relations, inputs and outputs, and internal states.

After your chosen supplier has delivered the system, you can compare it against your digital twin model to verify that they have in fact developed a CBI that behaves according to your specifications and system requirements. During the implementation phase, your digital twin model can also prove useful for training purposes, helping ensure that employees are onboard with the new system too. And once the system is ready and in service, the digital twin will enable you to maintain a detailed system overview and facilitate maintenance with greater ease.

Case study: Developing a digital twin for the Stockholm Metro

To understand what making the transition from a RBI to CBI system might look like in practice, we can recommend looking at a real life example. When it came time for the Stockholm Metro to upgrade their rail control system, they contacted us at Prover to assist with the transition. We created a digital twin for their current system and worked incrementally, testing to replace a part of the total system with the new system. Because we had the digital twin, we could do it with minimal interference. This case study was mainly focused on the technical aspects. Doing the replacement for real will require verification, validation, and assessment according to EN 50126.

Would you like to know more? Read the full case and step-by-step process.

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