Railway Turnout Guide

The railway turnout, also known as a switch or set of points, is a complex and highly engineered mechanical installation that enables a train to be guided from one track to another. It is arguably the most critical and high-maintenance component of any rail network, combining moving parts with fixed elements that must withstand immense dynamic forces. The safe and efficient operation of a railway turnout depends on a precise set of specifications governing its components, geometry, and materials. This technical guide explores the detailed specifications of a modern railway turnout, focusing on its core components and compatibility with various rail profiles.

Anatomy of a Railway Turnout: Core Components and Specifications

A complete railway turnout is a system of specialized components working in unison. Understanding each part’s function and specifications is key to understanding the system as a whole. The three primary sections are the switch panel, the closure area, and the frog assembly.

1. The Switch Panel

This is the “entrance” to the turnout, where the physical act of track switching occurs.

• Switch Points (or Switch Blades): These are the tapered, movable rails that guide the train’s wheels onto the straight or diverging path. They are machined from standard rail profiles (e.g., 136RE) to ensure a perfect fit against the stock rail. A critical specification is the sharpness and integrity of the point; any chipping or damage can pose a significant derailment risk.
• Stock Rails: These are the two fixed, outer rails of the main track. The switch point on each side rests against one of the stock rails. The stock rails are subjected to high lateral forces and side wear, especially in the area where the switch point makes contact.
• Switch Machine (or Point Machine): This is the powered or manual mechanism that throws the switch points from one position to the other. Its specifications include the force it can exert and the time it takes to complete a throw, which are critical for operational efficiency and safety.
• Slide Chairs: These are specialized plates located under the switch points. They must be kept clean and lubricated to allow the points to move smoothly from side to side while supporting them vertically.

2. The Closure Area

This section connects the switch panel to the frog assembly and establishes the geometry of the diverging path.

• Closure Rails: These are the curved rails that form the path between the heel of the switch points and the toe of the frog. Their curvature is precisely engineered based on the turnout’s frog number and design speed.
• Guard Rail (or Check Rail): Positioned opposite the frog, this short piece of rail performs a critical safety function. It engages the back of the wheel on one side of the axle to ensure the other wheel is safely guided through the flangeway of the frog, preventing it from taking the wrong path. The distance between the guard rail and the running rail is a critical, tightly-controlled specification.

3. The Frog Assembly

The frog is where the two running rails (one from the straight path, one from the diverging path) cross each other. It is the highest-impact area of the entire turnout.

• Frog Point: The physical point of the frog, typically made of high-strength manganese steel, which has the unique property of work-hardening under impact.
• Wing Rails: These rails run alongside the frog point, forming the flangeways that guide the wheel flange through the crossing.
• Frog Number: This is the most important specification of any turnout. It defines the angle of the turnout by the ratio of its length to its spread. A low frog number (e.g., No. 8) means a sharp, steep angle, while a high frog number (e.g., No. 20) means a shallow, gradual angle.

The Frog Number: The Defining Specification of a Railway Turnout

The frog number dictates the turnout’s overall geometry, length, and, most importantly, the maximum safe speed at which a train can traverse the diverging path. The choice of frog number is a direct trade-off between operational speed and the physical space the turnout occupies.

Advanced Turnout Designs for High Performance

To meet the demands of modern railways, several advanced turnout designs have been developed to increase speed and reduce maintenance.

Movable-Point Frogs

For high-speed turnouts (typically No. 20 and higher), the impact of a wheel jumping the flangeway of a traditional frog is too great. A movable-point frog eliminates this problem entirely.

• Design: Instead of a fixed point and gap, this frog features two short, movable point rails. These are connected to the switch machine and are aligned to create a solid, continuous running surface for whichever path is selected.
• Performance: By providing an unbroken rail path, movable-point frogs eliminate impact, noise, and vibration. This is the key technology that allows trains to pass through a railway turnout at speeds exceeding 160 km/h (100 mph).

Flange-Bearing Frogs

Used primarily in heavy-haul freight applications, a flange-bearing frog aims to reduce the high-impact forces on fixed frogs.

• Design: The floor of the flangeway is ramped upwards. As a wheel passes through, it is lifted slightly so that its weight is transferred from the tread to the flange for the brief moment it takes to cross the frog point.
• Performance: By providing continuous support for the wheel, this design dramatically reduces wear and tear on both the frog and the wheels, extending the life of these expensive components.

Compatibility with Rail Profiles and Fastening Systems

A railway turnout must be constructed from components that are fully compatible with the track it is being installed in. This means matching the rail profile and utilizing a robust fastening system.

Rail Profile Specifications

The rail profile (e.g., 136RE, 60E1) dictates the size, weight, and strength of the rail. All components of a turnout must be manufactured for a specific rail profile to ensure a perfect geometric match.

• Material Strength: The rail used in turnouts is often of a higher strength or hardness than the surrounding track, especially for the switch points and frog, which are high-wear components. Heavy-haul turnouts are almost exclusively built with head-hardened stock rails and solid cast manganese steel frogs to withstand extreme forces.
• Component Matching: A frog designed for a 136RE rail cannot be used with a 115RE rail. The height, head width, and fishing (the area under the head where joint bars connect) are all different. Using mismatched components would create a dangerous geometric flaw.

Rail Clips and Fastening Systems

The complex geometry of a turnout must be held rigidly in place. This is the job of the fastening system, which includes the rail clips, tie plates, and sleepers.

• High Clamping Force: The rail clips used within a turnout, especially around the frog and stock rails, must provide a high clamping force (toe load) to prevent any component from shifting under load.
• Specialized Tie Plates: A turnout uses a variety of specialized tie plates, such as slide chairs for the switch points and heavy-duty plates with rigid shoulders for the frog area.
• Integrated Sleeper Systems: Modern high-performance turnouts are built on a set of integrated, pre-stressed concrete sleepers (or “bearers”). These are manufactured to the exact geometry of the turnout, with a single long bearer often supporting multiple components. This ensures that the entire turnout maintains its precise alignment as a single, unified system.Q