Preparation of the User Requirements Specification for ETCS Level 2 System in Serbia - Experiences and Challenges

European Rail Traffic Management System (ERTMS) represents a set of contemporary technological solutions based on the Technical Specifications for Interoperability for Control-Command and Signalling Systems (TSI CCS). Within the ERTMS system there are two sub-systems: European Train Control System (ETCS) and Global System Mobile for Railways (GSM-R). Application level of ETCS system determines which type of trackside equipment shall be used, mode of communication between trackside and on-board equipment and processing of information within the trackside and on-board equipment in order to fulfill desired system requirements.

The railway corridors in the Republic of Serbia are not included in the list of the six major European ETCS corridors defined in clause 7.3.4 of Annex III of Technical Specifications for Interoperability for the Control Command and Signalling (TSI CCS) [1], nor in areas of special interest referred to in point 7.3.5. of the same, and the Republic of Serbia with the current status of joining the EU over an extended period is not eligible for the funds from the European Regional Development Funds, Cohesion Funds and TEN-T funds which impose installation of ETCS system in case of any upgrade of the existing signalling system.

However, preparation of the national URS for ETCS Level 2 system [2] was necessary in order to support the ongoing project for construction of new high-speed railway line Belgrade-Subotica-state border with Hungary. Although the Republic of Serbia is not yet a Member State of the European Union, the National Implementation Plan for ETCS (NIP) was prepared in parallel with URS pursuant to the article 6(4) and section 7.4.4 of the TSI CCS [1].

The main institutional stakeholders involved in the preparation of the URS were following:

•European Agency for Railways (ERA) - the European Union railway regulatory agency, with the task to regulate the European railway area and to promote the railway sector while maintaining the safety;

•“Serbian Railways Infrastructure JSC” (SRI) - Serbian national railway Infrastructure Manager;

•European Economic Interest Grouping (EEIG) ERTMS Users Group, consisting of suppliers of ERTMS equipment, thus enabling the safe, reliable, and interoperable railway network in Europe;

•Union Industry of Signalling (UNISIG) - the industrial consortium created to develop the ERTMS/ETCS technical specifications [3];

•ERTMS Deployment Management Team (ERTMS DMT) - the implementation support program unit, which provides technical and economical guidance to the stakeholders involved in ERTMS projects [4].

Since the subject is regulated by both TSI CCS [2] and national regulations, the methodology for URS preparation was based on the following assumptions:

•Main functions for the ETCS system functionality regarding the interoperability shall be defined according to the requirements from reference [1], as well as according to formal approach techniques defined in the references [5], [6], [7], [8], [9], [10];

•Specific national functions for the ETCS system functionality shall be defined having in mind basic requirements from references [11], [12].

The validity of the ETCS specification is usually considered as the most critical element for implementation in one country, and special caution measures were considered to secure full interoperability with requirements given in the reference [2].

Experiences that different EU member states have been identified in the previous years as a pre-requisite to deploying ETCS in their networks were implemented in order to avoid repetition of their mistakes.

After the wide scale consultations with all main stakeholders, it was decided that the provisions of the completed specifications shall be mandatory for all sections that belong to the Alpine-Western Balkan rail freight corridor, which represents the extension of the rail TEN-T network to the region of Western Balkan [13], [14], namely, they shall apply to the following railway line sections:

•Belgrade Centre - Sid - State border with Croatia - (Tovarnik);

•Belrade Centre - Junction “G” - Rakovica - Mladenovac - Lapovo - Nis - Presevo - State border with North Macedonia - (Tabanovce) [15];

•Nis - Dimitrovgrad - State border with Bulgaria - (Dragoman);

•(Belgrade Centre) - Resnik - Požega - Vrbnica - State border with Montenegro - (Bijelo Polje).

The provisions of the specifications are also mandatory for the railway section (Belgrade Centre) - Stara Pazova - Novi Sad - Subotica - State border with Hungary - (Kelebia), where the ETCS implementation was already in progress based on the intergovernmental agreement with the People's Republic of China [16], except in the points explicitly stated as an exception to these specifications, which will be the subject of a special agreement between the SRI and the Contractor.

Also, the provisions of the specifications shall be obligatory for other railway lines included in the National Implementation Plan for ETCS (see Figure 1), according to the guidelines given in the reference [17].

One of the first steps was to apply for national ETCS identification numbers (NID_C according to the definition in SUBSET 026-7 [1]), which was done in the prescribed procedure conducted by ERA. In the Table 1 are shown corresponding numbers assigned by ERA, which were officially published in the document [18].

In order to provide full backward compatibility between the trackside (infrastructure) and on - board (railway vehicles) components of ETCS, it was decided to implement the Baseline 3, Release 2 (SRS v3.6.0), system version X=1.0 (see Figure 2 originating from SUBSET 026-6-v3.6.0 [1]). Such solution enables that both Baseline 2 and Baseline 3 equipped trains can run on the railway lines equipped with ETCS Level 2 system.

For the estimation of the absolute braking distance was used the model given in the Annex F of the standard EN 14531-1:2019 - Railway applications - Methods for calculation of stopping and slowing distances and immobilization braking - Part 1: General algorithms utilizing mean value calculation for train sets or single vehicles [19]. This model was developed based on the practical experience on the French railways (SNCF), and can be used with assumption that braking force of the train is fully established (friction braking train). The model stipulates that absolute braking distance of the train is given as:

where, we have following meanings of variables:

$s_{grad}$: train braking distance on a given gradient of the related railway track (m)

$a_e$: equivalent train deceleration ($m/s^2$)

$t_e$: equivalent train driver response time (s)

$v_o$: initial speed of the train (m/s)

$v_{fin}$: final speed of the train (m/s)

$g_n$: standard gravity acceleration (9.81 $m/s^2$)

$i$: gradient of the related section of railway track (‰)

Using the above formulas and variations of certain parameters within the permitted limits used on the railway network of Serbia (e.g., $t_e\in$(1 to 3 s), $a_e\in$(0.7 to 1.1 $m/s^2$), $i\in$(-12.5 to +12.5‰), the characteristic diagrams given in the Figure 3 and Figure 4 below were obtained, which show that in most cases the value of the braking distance does not exceed 2500 m, and therefore it was adopted as a reference one. Similar values are defined in the reference [11].

According to Serbian national regulations, the maximum gradient of the railway track i equals -12.5‰. Maximum train acceleration $a_e$ for commercially available trains varies in the interval 0.7-1.1 $m/s^2$, and based on the experience from SNCF given in the reference [19], the driver response time shall be considered within the interval 1-3 s.

Basic considerations of this issue were elaborated in the reference [20]. National parameter T_NVCONTACT represents the maximum time during which the connection between ETCS on - board equipment and Radio Block Centre (RBC) can be interrupted, when operating in Full Supervision (FS) or On - sight (OS) mode, before the ETCS on-board equipment performs further operations according to the value of parameter M_NVCONTACT. Therefore, the proper determination of these two parameters is of crucial value for providing the overall safety of the ERTMS/ETCS system.

During the period in which the ERTMS/ETCS onboard communication with the RBC is interrupted, it is not possible to receive safety-critical ETCS messages (for example, the “Emergency Stop” command). If the value of the parameter M_NVCONTACT is set to “No reaction” then all trains that are not communicating with the RBC equipment can continue their driving routes until they reach the end of their Movement Authority (MA) or until communication is re-established.

From the safety perspective this is not an optimal solution, but its actual effect on safety depends upon the number and size of the areas where interruptions in radio coverage may occur, average length of MA, and the specific train route path.

Another safety related factor is the probability that already issued MA need to be shortened or revoked. In a system based on radio communication such as Global System Mobile for Railways (GSM-R), it is mostly possible to receive a confirmation that further driving is allowed. However, the safety implications for a certain location assessment can exist, where a train may be stopped at the unsuitable location or may be forced to brake very fast.

From a safety point of view, it is important that the value of T_NVCONTACT is as less as possible, since the risk of not implementing ETCS emergency stop will be lower. The lowest time shall not be less than the minimum period between the reception of two consecutive trackside messages, since this will produce the reaction defined in the variable M_NVCONTACT.

According to the Quality of Service (QoS) requirements for GSM-R system radio coverage (paragraph 6.4.1.1. from SUBSET-093 [1]), in case that error-free period is 20 s or longer, this shall enable the coverage probability of 95%, which is satisfactory for ETCS Level 2 applications for train speeds up to 220 km/h (which includes the national speed limit of 200 km/h).

Regarding the performance point of view, the most appropriate value of the parameter M_NVCONTACT shall be “No reaction”, since it will allow the train to continue to the end of the existing MA, even if the ETCS onboard equipment is disconnected from the RBC during operation in FS/OS mode. If, as mentioned above, the value “No reaction” is not appropriate, the usage of the value “Service brake” is more preferred in view of performance than that of “Train trip”, since the reception of a new message before the train stops will release the brakes and allow the train to proceed with driving.

Usage of “No reaction” value for M_NVCONTACT is generally assumed only when the frequency of radio disturbances and the need to shorten or revoke the MA is very low. Main factors in consideration of using the “No reaction” value for the parameter M_NVCONTACT are following:

•Depending on the length of already set MA and extent of the radio interruption, the train might be able to proceed for a significant path, without the effect on its progress;

•In case of Emergency Stops (SUBSET-026 section 3.10 [1]), the ERTMS may not completely depend on sending the Emergency Stop messages, but also on the possibility for GSM-R voice communication (Emergency calls), and therefore the failure of the radio system or radio signal reception interruption may result with loss of both data and voice information.

In the case of an area with radio interruption, its impact on operational performance depends on the duration and space. The localized failure (for example only one radio mast), can lead to the formation of “radio-hole”. If the value of T_NVCONTACT is low, not enabling the train to cross the “radio-hole”, then the train can be trapped until the fail is eliminated or the train leaves the “radio-hole”' with the onboard equipment set in an degraded operation mode.

From a point of view of operational performance, the most appropriate value of T_NVCONTACT shall be the largest possible one, since it will enable the railway operation less vulnerable to communication interruptions or failures. The real operational performance gain arising from larger values of T_NVCONTACT depends on the average length of MA maintained in front of the trains. It shall be noted that additional performance gain cannot be achieved by using values of T_NVCONTACT longer than time for which the MA is otherwise revoked (for example, when the train reaches the end of MA or MA expires).

On the other hand, usage of lower values for T_NVCONTACT can lead to a significant impact on operational performance, since the operation defined in the M_NVCONTACT will be executed before the receipt of potential “safe” message.

To conclude, the main principles for selection of the value of T_NVCONTACT are following:

•It shall be greater than the expected period in which the ETCS onboard equipment receives successive “safety”' messages;

•It shall be greater than the period required to recover from a “radio-hole”;

•It shall be greater than the time required to drive over “radio-holes”, caused by failures of a single radio mast or a single base transceiver station (BTS);

•It shall not be much greater than the typical time required to reach the end/limit of authority.

The values of T_NVCONTACT and M_NVCONTACT potentially can impact on the probability for operation in a degraded modes when recovering from the previous operations specified in the M_NVCONTACT. Degraded mode operation will further have an impact on operational performance.

Based on all previous considerations, the value “Service brake” was adopted for M_NVCONTACT and value of 20 s for T_NVCONTACT was adopted.

The SRI however can also consider a different value for prospective non-high-speed railway lines (for example “Train trip” or “No reaction”).

Basic considerations of this issue were elaborated in the references [11], [21]. Balise groups of signals must be placed in the manner that every possible movement in the direction toward signal passes exactly one balise group. Adopted distances of balise group from relevant types of signals are schematically shown in Figure 5 and elaborated in more details in the Table 2.

In certain cases (for example when leaving the ETCS zone), more than two balises in a group can be envisaged at a main signal.

Table 3 presents an overview of distribution for applied ETCS Level 2 packets transmitted between the ETCS track-side equipment and ETCS on-board equipment, based on the definitions from reference [22].

Table 4 presents an overview of distribution of applied ETCS Level 2 messages transmitted between the ETCS track-side equipment and ETCS on-board equipment, based on the definitions from reference [23].

To summarize, prepared URS should serve as a baseline for intended development directions when it comes to ERTMS/ETCS implementation in Serbia [24], [25], including all on-going projects/contracts covering the scope.

Based on the prepared specifications, the SRI has further developed additional documents required for ETCS Level 2 implementation, including annexes considering operational scenarios and RBC operator symbol catalogue, as well as updates of the existing Signalling Rulebook [26] and Traffic Regulation Rulebook [27] in respect to ERTMS/ETCS operation.

The future updates of these URS for ETCS shall include, inter alia, the requirements for ETCS Level 1, as well as eventual updates regarding new requirements from updated TSI CCS [2], whose adoption is expected during 2023.