Railway Track Change and Specifications

A railway track change, often called a turnout or switch, is a critical mechanical installation that guides trains from one track to another. The design and engineering behind these systems are fundamental to the operational flexibility and safety of any rail network. A turnout assembly consists of several key components, including a switch, a frog, and the connecting closure rails. Understanding the specifications of these parts, especially concerning standards set by organizations like the American Railway Engineering and Maintenance-of-Way Association (AREMA), is essential for constructing and maintaining reliable railway infrastructure.

Detailed Railway Track Change Components

A turnout is a complex assembly of specialized trackwork. The primary components work in concert to facilitate a smooth transition for rail vehicles. The main parts are the switch points, the stock rails, the frog, guard rails, and the hardware that holds everything together, such as rail clips and switch plates.

The switch itself comprises two tapered, movable rails known as switch points or point blades. These points are mechanically linked and pivot to align with one of two routes: the straight (mainline) track or the diverging (turnout) track. The fixed rails they press against are called stock rails. When a train approaches from the facing direction, the position of the switch points determines its path.

The Role of Frogs in a Railway Track Change

The frog is the component at the heart of a railway track change where the rail lines intersect. It allows the wheel flange to cross over the opposing rail. Frogs are typically single castings of high-strength manganese steel to withstand the immense impact and wear from passing trains. The divergence of a turnout is classified by its frog number.

Common turnout sizes include No. 8, No. 10, and No. 20. This number represents the ratio of divergence. For a No. 10 frog, the two diverging gauge lines will be one unit apart for every ten units of length measured from the theoretical point of the frog.

• No. 8 Turnouts: Often found in yards and low-speed areas.
• No. 10 Turnouts: A common size for mainline crossovers and turnouts, balancing space with moderate speed capabilities.
• No. 20 Turnouts: Used for high-speed mainlines where trains must maintain significant velocity through the diverging route.

The frog angle is inversely related to the turnout number; a smaller frog number corresponds to a larger angle and a sharper curve, which necessitates lower speeds. Larger frog numbers allow for more gradual curves and higher operational speeds.

Rail Specifications in Turnouts

The rail used within a turnout must meet strict specifications to ensure safety and durability. For most industrial and mainline applications, the rail section should be a minimum of 100 pounds per yard, with heavier sections like 115 lb/yd or 132 lb/yd used for high-tonnage or high-speed lines. The specifications for various AREMA rails are detailed and precise.

For instance, different rail profiles have specific dimensions for the head, web, and base. These dimensions are critical for ensuring compatibility with other track components like joint bars, tie plates, and rail clips.

AREMA Rail Specification Table (Example)

These specifications ensure uniformity and interoperability. In a turnout, the stock rail is a standard section, while the switch point is machined from a similar rail section, tapered to fit snugly against the stock rail. To protect the switch point tip, the stock rail is often planed or ground to create a recess, allowing the point to sit securely.

Switch Point and Stock Rail Details

The interaction between the switch point and the stock rail is a focal point of maintenance. A gap of as little as 1/4 inch between a closed switch point and its stock rail can create a risk of derailment. To prevent this, adjustable rail braces are used on the field side of the stock rail to provide robust lateral support.

The switch points themselves are manufactured with a vertical bend. This raises the head of the switch point slightly above the stock rail in the transition area. This design, known as a “riser,” prevents a wheel flange from catching the edge of the stock rail during a trailing-point movement, which could otherwise force the rails apart and cause a derailment. This elevation change is accommodated by specially designed switch plates or pads.

The Importance of Rail Clips in Track Stability

Rail clips are a vital part of the rail fastening system, responsible for securing the rail to the tie plate and, by extension, to the crosstie. While traditional cut spikes are still in use, modern elastic rail clips offer superior performance, especially in high-stress areas like turnouts. Elastic clips exert a constant clamping force on the rail base, preventing both longitudinal movement (rail creep) and lateral shifting.

In the context of a railway track change, this stability is paramount. The dynamic forces exerted by a train as it navigates a curve or passes through a frog are significant. Rail clips help maintain the precise track gauge and alignment necessary for safe operation. Without adequate fastening, the rails could spread, leading to a wide-gauge condition and increasing the risk of derailment.

There are many types of rail clips, each designed for specific applications and rail sections. Some common examples include:

• Pandrol ‘e’ Clip: A widely used clip known for its ease of installation and reliability.
• Safefix Clip: Offers high clamping force and is often used in heavy-haul or high-speed applications.
• Vossloh SKL Clip: Another popular tension clamp system that provides long-term stability.

These clips are designed to work as part of an integrated system with the tie plate and anchors. In a turnout, the layout of tie plates (switch plates) is complex, accommodating the switch points, stock rails, and closure rails. Rail clips ensure that each of these components remains securely fastened, absorbing vibrations and maintaining the integrity of the track structure. The constant clamping force helps prevent the loosening of bolts and other components, reducing maintenance needs and enhancing the overall safety of the railway track change.