The Physics of Safety: UIC 716 Track-Vehicle Force Limits

Definition and Scope

UIC Leaflet 716 titled “Maximum permissible forces for the interaction between track and vehicle,” defines the critical safety limits for the dynamic forces generated where the steel wheel meets the steel rail. It serves as the fundamental reference for determining whether a railway vehicle is safe to operate without causing derailment or permanent damage to the track superstructure.

While modern homologation often refers to the harmonized standard EN 14363 (or UIC 518 for testing methodology), UIC 716 established the foundational values for running safety. It acts as the mathematical referee, balancing the vehicle’s desire to move fast with the track’s physical limitations.

Key Technical Parameters

UIC 716 focuses on three main vectors of force that occur during train movement, particularly in curves.

1. Derailment Coefficient (Y/Q Limit)

The most famous criterion in railway dynamics. It measures the ratio between the lateral force (Y) pushing the wheel flange against the rail and the vertical force (Q) pressing the wheel down.

• Nadal’s Criterion: The standard typically sets a limit (e.g., Y/Q < 0.8) to prevent the wheel flange from climbing up and over the rail head. If the lateral force is too high relative to the vertical weight, the wheel climbs, leading to derailment.

2. Track Shifting Force (Prud’homme Limit)

Even if the wheel doesn’t climb the rail, excessive lateral force can physically push the entire track panel (rails + sleepers) sideways through the ballast.

• Formula: The limit is often expressed as a function of the axle load. For example: (Sum of Y forces) < k * (10 + 2Q/3). This ensures the vehicle does not distort the track geometry, which would cause maintenance issues or accidents for subsequent trains.

3. Vertical Forces and Impact

UIC 716 also limits maximum vertical dynamic loads (Q) to prevent rail breakage and degradation of the subgrade. High dynamic impacts (e.g., from wheel flats) are strictly regulated.

Comparison: Types of Forces

Understanding the distinction between these force limits is vital for vehicle designers and track engineers.

Relation to Modern Standards

UIC 716 provides the limit values, but it works in tandem with UIC 518, which describes the testing method to verify these values. Today, European Norm EN 14363 integrates both the limits from UIC 716 and the methods from UIC 518 into a single comprehensive standard for rolling stock homologation.