Railroad Engineering Basics
Learning Objectives
- Understand the components and functions of the railway track structure system.
- Differentiate between the superstructure and substructure.
- Identify common rail types and the mechanism of wheel conicity.
- Comprehend the restrictive geometric design principles of railways.
- Evaluate how track gauge and wheel conicity safely guide trains through curves.
The Track Structure System
Unlike highways where rubber tires contact a wide pavement surface, railways rely on the precise interaction between steel wheels and steel rails. This fundamental difference dictates a unique track structure designed to distribute immense concentrated loads (point loads from train wheels) safely down to the natural soil.
Superstructure
The upper part of the railway track consisting of rails, fastenings, and ties.
Substructure
The foundational part of the railway track consisting of ballast, subballast, and subgrade.
A conventional railway track consists of these two main parts: the Superstructure and the Substructure. They work together to distribute immense concentrated loads safely down to the natural soil.
Interactive Simulation
Interact with the simulation below to explore the components of a standard railway track structure.
Railway Track Structure
Controls
Info
The track structure distributes immense point loads from the wheels safely to the subgrade.
Track Gauge
The perpendicular distance between the inner faces of the two rails. In North America and much of the world, Standard Gauge is . Maintaining exact gauge is critical to prevent derailments.
Components of the Superstructure
Rails
Ties (Sleepers)
Types of Rail Sections
Throughout history, different rail shapes have been used. The most common types are:
Rail Sections
- Flat-footed Rail (Vignoles Rail): The universal standard today. It has a flat base that can be fastened directly to ties (using base plates) without needing supporting chairs. It is stable and economical.
- Bull-headed Rail: Once common in the UK. The head is slightly thicker than the foot. It cannot stand upright on its own and requires heavy cast-iron "chairs" to hold it to the ties.
- Double-headed Rail: The earliest form, designed with a symmetrical head and foot with the idea that when the top wore out, the rail could be flipped over. This proved impractical, as the bottom also wore out by grinding against the chairs.
Creep of Rails
A major maintenance issue in continuous welded rail (and jointed rail) is creep.
Creep
The longitudinal movement of the rails relative to the ties. It is primarily caused by the wave action of the rail yielding under the moving wheel loads, dragging the rail forward in the direction of traffic. It can also be caused by severe braking forces. If uncontrolled, creep causes track buckling or joint pull-aparts. Anti-creep devices (anchors) are fastened to the base of the rail to bear against the ties and stop this movement.
Components of the Substructure
Ballast
Subgrade
Railway Track Geometry Principles
The geometric design of railways is generally much more restrictive than highways due to the lower friction between steel wheels and rails, and the sheer mass and length of trains.
Superelevation (Cant)
The banking of the outer rail on a curve to counteract centrifugal force.
Cant Deficiency
A condition where the actual cant applied is a compromise, resulting in faster trains experiencing some outward push.
Vertical Alignment
The longitudinal profile of the track.
Geometric Design Principles
- Horizontal Curves: Trains cannot negotiate sharp curves. Minimum radii are much larger than for highways.
- Superelevation (Cant): Because trains travel at varying speeds (freight vs. passenger), the actual cant applied is often a compromise, resulting in a cant deficiency for faster trains (they experience some outward push) and an excess for slower trains.
- Vertical Alignment: Trains are extremely sensitive to steep grades. A grade on a railway is considered steep and significantly reduces the hauling capacity of a locomotive. Vertical curves connecting grades are very long and flat.
Coning of Wheels
The tapering of railway wheels at an angle (usually 1:20) that allows a self-centering steering mechanism when a train enters a curve.
Wheel Conicity Mechanism
The interaction between wheel and rail is paramount. The wheel tread is typically conical (tapered), and the rails are canted slightly inward (usually at a 1:20 or 1:40 slope). This conicity creates a self-centering steering mechanism: when a train enters a curve, centrifugal force pushes the wheelset outward, causing the outer wheel to ride on a larger diameter of its cone and the inner wheel to ride on a smaller diameter. This difference in rolling circumference naturally steers the solid axle around the curve, minimizing flange contact and wear.
Wheel Conicity and Rail Fastenings
The intricate interaction between the wheel and the rail is the basis of railway mechanics.
Coning of Wheels
Rail Fastening Systems
- Railway Engineering manages intense point loads from steel wheels through a layered structure down to the subgrade.
- The Superstructure consists of rails (for guidance/support) and ties (for gauge maintenance/load transfer).
- The Substructure consists of ballast (for drainage/stability) and the natural subgrade foundation.
- Maintaining an exact standard Track Gauge () is critical for safe operation and avoiding derailments.
- Modern rails are typically continuous welded rail (CWR), often using the stable flat-footed profile.
- Geometric Design for railways is highly restrictive, requiring massive radii, very flat grades, and precise superelevation (cant).
- Wheel Conicity provides a self-steering mechanism that allows solid axles to naturally negotiate curves without slipping or excessive flange wear.
- Modern elastic rail fasteners absorb vibration, resist creep, and are essential for clamping continuous welded rail.