UIC Leaflet 737-4: Sensitivity of Track Circuits – Impact of Sanding

What is UIC Leaflet 737-4?

UIC Leaflet 737-4 is a safety-critical document titled “Measures to be taken to ensure the good functioning of track circuits.” It addresses one of the most significant conflicts in railway engineering: the need for Sanding to improve Adhesion versus the need for electrical conductivity to ensure Operational Safety.

Track Circuits rely on the train’s wheels and axles to create a short circuit (shunt) between the two rails. When a driver applies sand to the rails to prevent wheel-slip during acceleration or braking, the layer of sand can act as an insulator. If the insulation is too thick, the signalling system may “lose” the train, falsely indicating that the track section is clear.

Technical Limits for Sanding Systems

To prevent “loss of shunt,” UIC 737-4 provides strict Technical Delivery Conditions for the design and use of sanding equipment on Rolling Stock:

• Sand Flow Rate: Specifies the maximum amount of sand that can be discharged per minute. Excess sand increases the risk of creating a non-conductive barrier.
• Sand Quality: Defines the grain size and chemical composition of the sand. Fine dust can create a more compact insulating layer than standardized grit.
• Interlocking with Speed: Requirements for systems that automatically reduce or stop sanding as the train speed decreases, as the risk of insulation is highest at low speeds or when stationary.

Impact on Signalling and Infrastructure

For Infrastructure Management, the leaflet dictates how track circuits should be adjusted to maintain sensitivity:

• Voltage Thresholds: Setting the electrical parameters of the track circuit high enough to “punch through” minor contaminants but low enough to remain safe.
• Alternative Detection: Guidelines for using axle counters in areas where heavy sanding is frequent (e.g., steep gradients), as axle counters are immune to the insulating effects of sand.
• Maintenance Protocols: Requirements for cleaning rails in areas where sand accumulation is detected.

Operational Safety and “Loss of Shunt”

A “Loss of Shunt” is a high-severity incident. UIC 737-4 outlines the operational procedures to mitigate this risk:

• Emergency Sanding: While routine sanding is restricted, the leaflet allows for unrestricted sanding during emergency Brake Application, prioritizing stopping distance over track circuit sensitivity in life-safety scenarios.
• Detection Monitoring: Systems that alert the dispatcher if a track circuit shows intermittent occupancy (flickering), which is a classic symptom of sand interference.

Comparison: Sanding Impact on Detection Systems