TRADITIONALLY controlled limitations on the distances between signals and points and the point-to-point cabling of the field elements has underpinned the cost of constructing and operating electronic interlockings.

These technical specifications, which have previously been difficult if not impossible to change, limit an interlocking area to a maximum of 13km. As a result remote computer locations are deployed to cover longer distances, which require the construction of numerous equipment buildings to house wayside interlocking equipment, and its associated power supply and air-conditioning systems.

Such installations inevitably increase costs, with cabling, cable ducts and cable distribution racks accounting for 20-30% of the cost of the entire interlocking.

SinetSwiss Federal Railways (SBB) is searching for cheaper, more adaptable solutions for operating its field elements with the aim of introducing an adaptable interlocking architecture, which reduces cabling costs, improves or at the very least offers the same level of availability, while providing an open-standard system.

Following an invitation to tender in 2010, Siemens responded to SBB's demands for innovative solutions by offering its Sinet interlocking architecture.

Sinet is designed to offer real-time and high-availability connections between decentralised field elements in interlocking systems with the aim of reducing costs. The system's built-in redundancy mechanism and an intelligent communication component, which is specially developed for the railway environment, works to prioritise operational data while providing the high system availability required.

This technology was successfully introduced for the first time last autumn in an SBB Simis W electronic interlocking at Sevelen and a further implementation will go live this month in Tösstal.

In order to prove the technology in front line service, initially only the signals and level crossings in the interlocking were operated via a data network. However, it is possible to achieve the required increases in efficiency by deploying an integrated solution across all field elements. The higher-level concepts and standards were therefore defined at the same time that the SBB project was implemented thus facilitating a quick and low risk step-by-step introduction.

Sinet-fig1Figure 1 shows a diagram of the structure of an interlocking using the Sinet architecture. Element controllers (EC) are located close to the field elements they operate and monitor, with detailed diagnostic information used to efficiently manage and rectify any faults.

The ECs are connected to both the communication bus and a separate power source, which is an important feature in adapting the system to specific situations and for retaining maximum reliability independent of the communication system.

A redundant connection links the ECs with the interlocking via the communication bus. The redundancy is implemented by the standardised Parallel Redundancy Protocol (PRP) specified in IEC 62439-3.

Figure 2 shows a diagram of the communication architecture. The element controllers are connected to the communication network by a Sinet Communication Unit (SCU) via two independent network paths. A data packet sent by a terminal unit is always transmitted over both physically separate network paths. At the receiving end, the first valid data telegram is accepted and the second redundant telegram discarded. This means that any loss of Sinet-fig2redundancy is diagnosed immediately, while communication between the end points is maintained.

As the PRP used is located beneath the TCP/IP stack all users benefit from the redundancy provided by the PRP (Figure 3). As a result, other services such as diagnosis via the Simple Network Management Protocol (SNMP) and network camera streaming are also connected redundantly to the corresponding indoor equipment. Consequently, in the event of a single fault in the communication infrastructure these services remain available.

In addition to operational data like commands and messages, information such as diagnostic data is also transmitted through the Sinet communication network. Prioritising the operational data means it is transmitted in real-time between the various communication end points. In contrast, conventional IP data telegrams are handled with a lower priority and cannot disturb real-time operational dataflow, a cornerstone of the Sinet philosophy.

Figure 4 shows a layer model of failsafe communication between the interlocking computer and the element controller. Safety-related applications are implemented in the form of technical processes at both end points superseding the need to meet safety requirements for the communication components as outlined in EN 50126.

The SCU supports the TCP/IP stack, and with the standardised PRP provides redundant transmission of the user data. If safety protocols with integrated redundancy mechanisms, such as Redundant Safe Transport Application (RaSTA), are used in the safety layer, they are also supported by Sinet. Here again, Sinet allows the redundant transmission of non-vital telegram traffic, such as diagnostic information. Thanks to the use of standardised interfaces and protocols throughout, it is possible to connect a large number of indoor and outdoor components to the Sinet architecture, giving it huge scope for future development.

Sinet-fig34In addition to transparent transmission of diagnostic data from the element controllers, Sinet also provides detailed diagnostic information about the status of the communication network. By employing a higher-level diagnostic system such as the SNMP typically used in network technology, it is possible to centrally monitor the network's status. The web server integrated in the SCU can also analyse detailed information such as a line's bit error rate. And through appropriate routing, it is possible to make the diagnostic data available at any location in the customer's network.

Because Sinet is compatible with today's interlocking infrastructure, migration can take place step-by-step or only partially, if necessary. For example, in the Sevelen project, the signals and level crossings were connected by Sinet technology, while the other field elements were integrated with conventional point-to-point connections. Moreover, it is also possible to assign existing interlocking areas already based on Sinet architecture to another interlocking by reconfiguring the network.

The tender for the Sevelen interlocking indicated that it would be an optimal pilot deployment for Sinet, with the size, equipment and commissioning date ideal prerequisites for a trial.

The Sevelen construction project and the Sinet development project were managed separately by SBB and Siemens. However, both project teams were in continual contact throughout the process.

Sevelen is operated as a remote interlocking from the Simis W type used in Buchs. In Sevelen, 12 signals are connected via Siemens' MSTT modular control actuators and three level crossings via LCM200 with Sinet communication architecture. The system includes two field rings, each starting from an interlocking building, with the larger of the two rings covering a distance of around 5km.

In order to minimise risks, the system was constructed redundantly and tested using both new and conventional cabling. This meant that if a difficult technical problem occurred with Sinet, it would be possible to immediately revert to the conventional solution. Development and construction deadlines were met, and commissioning took place as planned at the end of October 2013.

Following on from the Sevelen application, the first stage of the Tösstal interlocking will go into operation this month. The new Simis W type interlocking will control around 30km of a single track line with seven junction stations which require interlockings, and four intermediate stations with no interlocking requirements.

The system includes 66 MSTT signals and 30 level crossings with LCM200, which are connected by 14 field rings. The field rings in Tösstal will be connected to the interlocking computers for the first time using SBB's redundant SDH network to construct multi-station field rings.

With Sinet encompassing both outdoor equipment networking and the use of telecommunication networks in the electronic interlocking, the railway's engineers have to develop a strong understanding of both areas in order to properly realise the benefits of low cost and simpler installations and the lower maintenance requirements offered by Sinet.

Integrating each technical department from the start of the process was essential in developing and introducing an appropriate training programme. The agreed procedure for configuration and assignment of the IP addresses is a typical example of how SBB and Siemens' technical departments jointly defined the specifications.

The advance testing of the actual network technology on a particularly heavily-used SBB line in non-safety critical circumstances has allowed the development of the safety standards required to achieve the technology's desired robustness and performance capability. Following close cooperation between Siemens' and SBB's technical departments, the project is expected to enter service with no major difficulties.

Controlling signals and level crossings is regarded as the first step in the deployment of Sinet. SBB will continue to pursue its target of operating all field elements using the new concept by 2018, and is also looking to collaborate with German Rail (DB), which is pursuing the same goals, on the project.

By optimising the data communication and interlocking architecture power supply, the interfaces between the operator's infrastructure and the electronic interlocking will have more layers than current conventional proprietary approaches. An in-depth understanding of the operator's modified systems, such as the network backbone and the corresponding operational processes, are very important for developing a robust concept.

SBB and Siemens realised this at an early stage and have consistently exploited the corresponding synergies. The successful introduction of Sinet is demonstrating the advantages of connecting safety critical equipment such as signals and level crossings to a modern IP network. However, even more principles and standards will have to be specified and implemented in order to successfully implement a completely new interlocking architecture.

SBB's initial installations have shown that there is real potential for Sinet to bring the costs of signalling upgrades down significantly without compromising safety. Furthermore, the potential for additional capabilities to be added to Sinet is huge, making it both upgradeable and future proof for infrastructure managers as they strive to deliver a completely new interlocking architecture. Indeed with further rollouts in Switzerland expected, it is likely that many of these developments will be realised on SBB's network.