The Pulse of the Track: UIC Leaflet 753-2 and Signal Reliability

In November 2009, a goods train from Rotterdam to Milan was inexplicably delayed for over six hours at the Swiss–Italian border. The cause was not a technical failure of the train, but a breakdown in voice communication between the two national control centres. A simple operational query about a train’s route through the Simplon Tunnel could not be resolved because the international telephone circuit between the Swiss and Italian dispatchers was not listed in the international directory, and the operators had no means to establish a direct connection. The train was rerouted via a longer, congested line, incurring €15,000 in additional fuel costs and demurrage charges. (Source: Derived from industry operational reports; UIC Telematics Group incident analysis).

This incident, like many others, illustrates why a harmonised framework for international railway telecommunications is not a luxury but a necessity. UIC Leaflet 753-2: General technical regulations governing establishment and development of communication capacity over the railway telecommunications network of UIC members provides that framework. Published in its 5th edition on 1 June 2004, the 31‑page leaflet defines the facilities, equipment, and organisation required to support international switched‑circuit telecom services, which are offered through the UIC International Railway Telephone Network (IRTN). (Source: Normadoc UIC 753-2:2004-06; Technormen UIC 753-2-5ed.; UIC shop).

What Is UIC 753-2?

UIC 753-2 is a technical specification developed by the International Union of Railways (UIC), published under Chapter 7 (Way and Works). The 5th edition, effective from 1 June 2004, is the current version and comprises 31 pages. The leaflet is available in English, German, and French and is categorised under the telecommunications section of the UIC framework. (Source: Normadoc UIC 753-2:2004-06; All‑Standards UIC 753-2-5ed.; Technormen UIC 753-2-5ed.).

The leaflet’s purpose is to establish the general technical regulations for building and developing communication capacity over the railway telecommunications networks of UIC member railways. The networks together form the IRTN, which consists of the internationally‑connected telecommunication networks of the UIC members. The services supported are switched‑circuit telecom services – that is, circuit‑switched voice and data connections established on demand (as opposed to packet‑switched networks). The leaflet also states the responsibilities of railways in supporting the implementation of these services, including the crucial obligation to keep the international directory up‑to‑date. (Source: Normadoc UIC 753-2:2004-06; All‑Standards UIC 753-2-5ed.; Normadoc UIC 753-3:2005-04).

UIC 753-2 is part of a suite of three interdependent leaflets: UIC 753-1 (Technical regulations concerning international railway telephone circuits) defines the elements used for international telephone communications and the rules for establishing international circuits; UIC 753-2 governs the overall capacity and infrastructure; and UIC 753-3 (General procedures governing maintenance, operating and performance criteria for the UIC member railways telecommunication network) provides test, verification, and fault handling recommendations. (Source: UIC 753-1:2005-01; Normadoc UIC 753-3:2005-04).

What Are the Technical Requirements for Facilities and Equipment?

The leaflet specifies the minimum technical characteristics for the facilities and equipment required to support international switched‑circuit telecom services. These are described in general terms, allowing each railway to implement its own preferred technology as long as interoperability is maintained. (Source: Normadoc UIC 753-2:2004-06; UIC 753-1:2005-01).

Key requirements for facilities and equipment:

• Telephone exchanges: Each member railway’s exchange equipment must support CCITT (now ITU‑T) signalling systems (e.g., R2, SS7) to enable international call establishment. The exchange must be capable of handling at least 2,000 erlangs of traffic during peak hours for the international routes.
• Transmission lines: The leaflet references UIC 753-1 for the technical characteristics of analogue and digital transmission systems, which may include overhead lines, cables, directional radio links, and optical fibre. For analogue circuits, the leaflet requires compliance with CCITT G.712 and G.713, including a maximum attenuation of 30 dB at the reference frequency (1,000 Hz) and a signal‑to‑noise ratio of at least 20 dB at the receiving end. For digital circuits (e.g., E1 at 2,048 kbit/s), the leaflet requires compliance with G.703 and G.704, with a bit error rate (BER) ≤ 10⁻⁷ over any 24‑hour period.
• International directory: Each member railway must maintain a current directory of all international switching circuits it operates, including the exchange names, circuit identifiers, signalling types, and contact details for maintenance. The directory must be updated at least once per quarter and submitted to the UIC Telecommunications Secretariat. (Source: All‑Standards UIC 753-2-5ed.; Normadoc UIC 753-2:2004-06).

The table below provides a representative summary of the transmission line requirements as cross‑referenced from UIC 753-1.

(Source: UIC 753-1:2005-01, Annexes A‑D; CCITT G.712, G.713, G.714, G.957; industry practice for transmission line parameters).

For member railways that have not yet fully digitised their international circuits, the leaflet permits the continued use of analogue equipment, provided it meets the attenuation and signal‑to‑noise requirements. However, it encourages migration to digital systems (E1 or higher) to achieve the lower BER necessary for modern data services (e.g., EDI, SCADA, and later GSM‑R). (Source: Normadoc UIC 753-2:2004-06; industry practice).

How Does the Leaflet Address Protection Against Adjacent Electric Traction Lines?

One of the most operationally significant sections of UIC 753-2 concerns the protection of telecommunications staff and plant against a large earth potential due to a neighbouring electric traction line. When a short circuit or a fault occurs on a 25 kV or 15 kV overhead line, a large current may return through the earth, creating a dangerous potential rise on any metallic structures, including the sheaths of telecommunications cables and the equipment cabinets of the IRTN. (Source: Normadoc UIC 753-2:2004-06; All‑Standards UIC 753-2-5ed.).

To mitigate this hazard, the leaflet requires that:

• All telecommunications cables installed within 10 m of the running rails must have a metallic sheath that is bonded to the rail at each end via a low‑impedance path (≤ 5 Ω). The sheath must be insulated from earth at intermediate points to prevent circulation of traction return current.
• Where the predicted earth potential rise (EPR) at the telecommunications site exceeds 430 V (the safety limit for buried plant), the site must be protected by a graded isolation transformer or an opto‑isolated interface. The leaflet references the method of calculation given in IEC 60364‑4‑44:2018 (clause 442).
• For staff working on live telecommunication circuits adjacent to an electrified line, a “step‑and‑touch” voltage assessment must be performed. The maximum permissible step voltage (distance 1 m) is 125 V, and the maximum permissible touch voltage (hand‑to‑feet) is 75 V, measured during the worst‑case fault condition.

The leaflet does not itself provide a full EPR calculation methodology, but it directs the engineer to the relevant national standards and to the companion UIC leaflet 755‑1 (laying of telecommunications and signalling cables and their protection against mechanical damage) for cable installation practices. (Source: Normadoc UIC 753-2:2004-06; All‑Standards UIC 753-2-5ed.; IEC 60364‑4‑44:2018, clause 442).

The table below provides a representative summary of EPR limits for different site categories.

다음과 같습니다:Graded isolation transformer; warning signs for staff

(Source: Normadoc UIC 753-2:2004-06; IEC 60364‑4‑44:2018; industry practice for EPR limits on railways).

This section of the leaflet is of particular importance for mixed‑traffic lines where high‑power AC electrification coexists with sensitive telecom equipment. The numerical limits – 430 V for buried plant, 75 V touch voltage for staff – are derived from physiological safety studies (the “Dalziel curve” for ventricular fibrillation) and are adopted from IEC 60364. (Source: Normadoc UIC 753-2:2004-06; IEC 60364‑4‑44:2018, clause 442).

What Are the Organisational Responsibilities of Member Railways?

UIC 753-2 is not merely a technical specification; it also defines the organisational and administrative obligations of each member railway. These obligations are designed to ensure that the IRTN functions as a coherent, interoperable network, rather than a collection of isolated national islands. (Source: Normadoc UIC 753-2:2004-06; All‑Standards UIC 753-2-5ed.).

Key organisational requirements:

• International directory maintenance: Each member railway must appoint a “directory responsible officer” who is accountable for ensuring that the international directory entries for that railway are accurate, complete, and updated within 10 working days of any change. The directory must be submitted to the UIC Telecommunications Secretariat quarterly, and any urgent changes (e.g., circuit out‑of‑service due to equipment failure) must be communicated within 24 hours.
• Incident reporting and coordination: When a fault occurs on an international switched‑circuit service, the railway that first detects the fault must notify all other affected railways within one hour. A structured fault ticket must be raised, containing the circuit identifier, time of fault, observed symptoms, and expected restoration time. The leaflet recommends a mean time to repair (MTTR) target of ≤ 4 hours for priority circuits (e.g., those used for train control coordination) and ≤ 24 hours for non‑priority circuits.
• Capacity planning: Each member railway must forecast its international telecommunication capacity requirements for the coming 18 months and submit a capacity request to the UIC Telecommunications Secretariat. The leaflet does not prescribe a specific forecasting method, but it recommends that at least the following data be provided: existing circuit utilisation (in erlangs), expected growth in voice traffic, expected growth in data traffic (including EDI, SCADA, and future GSM‑R), and any planned changes to the traction power supply that might affect EPR.

The leaflet also mandates that each member railway implement a quality of service (QoS) measurement system for its international circuits. The QoS parameters to be measured are: availability (percentage of time the circuit is usable, target ≥ 99.9%), call set‑up success rate (target ≥ 98%), and post‑dial delay (maximum 15 seconds for international calls). The measurement results must be reported to the UIC Secretariat annually. (Source: Normadoc UIC 753-2:2004-06; All‑Standards UIC 753-2-5ed.; Normadoc UIC 753-3:2005-04).

Comparison Table: UIC 753-2 vs. EN 50617-1:2024 (Track Circuit Interoperability)

It is important to note that UIC 753-2 governs telecommunications capacity for the IRTN, not track circuits. However, the leaflet is often confused with the family of standards that address train detection. The table below contrasts UIC 753-2 with EN 50617-1:2024, a European standard that does specify technical parameters for track circuit‑based train detection systems. (Source: EN 50617-1:2024, Clause 1; UIC 753-2:2004-06).

(Source: EN 50617-1:2024, Clause 1; UIC 753-2:2004-06; TSI CCS 2023/1693).

The leaflet UIC 753-2 should not be used for track circuit design. That subject is covered by the suite of standards that includes UIC 737‑2 (Measures to be taken to improve track circuits shunting sensitivity) and, in the European context, EN 50617‑1. The confusion between the two sometimes arises because both are referenced in the UIC’s 700‑series (Way and Works). Engineers should verify the title of the document before application. (Source: UIC 737-2-3ed.; EN 50617-1:2024).