Staying on Track: UIC Leaflet 645 and the Science of Curve Negotiation

What is UIC Leaflet 645?

UIC Leaflet 645, titled “Conditions imposed on tractive units used in international traffic regarding the negotiation of curves” (French: Bedingungen, die an die im internationalen Verkehr eingesetzten Triebfahrzeuge bezüglich des Befahrens von Gleisbögen gestellt werden), is a safety-critical technical standard. It defines the mechanical capabilities a locomotive or power car must possess to safely navigate the twists and turns of the European railway network.

While a train on a straight track is relatively stable, entering a curve introduces complex physical forces. The flanges of the wheels push against the rail, and the vehicle body wants to continue straight due to inertia. UIC Leaflet 645 provides the engineering “rules of the road” to ensure that these forces do not cause the vehicle to climb over the rail (derailment) or damage the track structure, even on tight curves found in older depots or mountainous routes.

Key Technical Specifications

The leaflet addresses the geometry of the vehicle’s running gear (bogies and axles) and its interaction with track infrastructure.

1. Curve Inscription (Geometric Capability)

The standard mandates the minimum curve radius a vehicle must be able to negotiate.

• Main Line: Vehicles must handle standard curves (typically down to 150m radius) at speed without excessive wear.
• Depots and Sidings: Critical for “Tractive Units,” they must often navigate extremely tight curves (down to 80m or 90m radius) found in maintenance workshops and shunting yards.

2. Security Against Derailment (Twisted Track)

Real-world track is never perfectly flat. “Twist” occurs when one rail rises or falls relative to the other over a short distance (e.g., at the entry to a banked curve). UIC 645 specifies that the vehicle’s suspension must be flexible enough to keep all wheels on the ground. If a wheel unloads too much (loses contact pressure), the risk of the flange climbing the rail increases significantly.

3. Projection and Overhang

When a long vehicle turns a corner, its center cuts the corner (inswing) and its ends swing out (outswing). UIC 645 limits these geometric excursions to ensure the locomotive does not strike signals, platforms, or trains on adjacent tracks.

Comparison: UIC 645 vs. Modern EN Standards

For decades, UIC 645 was the bible for vehicle authorization. In recent years, its principles have been absorbed and expanded by European Norms (EN), specifically EN 14363.

Why is this important for “Small Wheels”?

A specific section of UIC 645 (often referenced in safety dossiers) deals with vehicles equipped with small diameter wheels (e.g., low-floor trams or specialized maintenance vehicles). Smaller wheels have a higher risk of dropping into the “gap” at switches and crossings (frogs). UIC 645 sets strict limits on the wheel profile and axle guidance to prevent these wheels from jamming or derailing at complex junctions.