Track Structure — Rails, Sleepers, Ballast & Fastenings

The track as a structure

A railway track is the structure that carries the train and guides its wheels. It is built up in layers: rails carry the wheel load and guide the wheel; fastenings hold the rails to the sleepers; sleepers spread the load and keep the gauge; ballast spreads the load further and provides drainage and adjustment; the formation (subgrade) carries the whole load to the ground.

At each stage the load is distributed over a larger area, so that the pressure on the ground is a small fraction of the wheel contact pressure.

Rails

Functions

• Provide a hard, smooth and continuous running surface with low friction.
• Guide the wheels laterally.
• Bear the wheel load and transmit it to the sleepers, acting as a continuous beam on elastic supports.
• Serve as the return conductor for signalling currents and, in electric traction, for the traction current.

Rail section

The standard is a flat-footed (Vignoles) rail consisting of head (wearing surface), web and foot. The main rail sections on Indian Railways are the 52 kg/m and 60 kg/m rails (with high-tensile 90 UTS steel; 72 UTS in some lines); older sections such as 90 R (45 kg/m) survive on secondary lines. Rail weight per metre is selected from the axle load and speed — heavier rails for heavier loads and higher speeds. The head must be of sufficient depth for wear, the web thick enough to resist buckling and corrosion, and the foot wide to give lateral stability and a large bearing area on the sleeper.

Rail length, welding and LWR

• Rail length: standard 13 m (BG). Rails were laid with fish-plated joints, which are weak spots, noisy, and require heavy maintenance.
• Welded rail panels: rails are welded into SWR (short welded rails, up to about 3 rail lengths), long welded rails, LWR, and continuous welded rails, CWR (a section between switches).
• Welding: flash-butt welding in a plant (the best quality) and alumino-thermic (AT) welding on site. Welding eliminates joints, giving a smoother ride, lower maintenance and less wear on wheels and track.

Long welded rail (LWR) — a rail long enough that its middle portion does not move with temperature (it is held by the fastenings and ballast), and only the ends (breathing lengths, about 100–150 m) expand and contract. The temperature stress in the fixed portion is $\sigma =E\alpha \Delta T$, which for a temperature rise of 40 °C is about $2.1\times 10^5\times 1.15\times 10^{-5}\times 40\approx 97$ N/mm². To keep LWR safe, the rail is laid at a stress-free (neutral) temperature in the mid-range so that it does not buckle in summer or fracture in winter; sleepers of concrete with strong fastenings and a full ballast section provide the lateral and longitudinal restraint. Destressing is done when the laying temperature is off the desired range.

Rail defects

Common defects include wear (head, side wear on curves), corrugation, rolling contact fatigue cracks (head checks, squats), wheel burns, shelling and fractures at bolt holes or welds. Rails are inspected with ultrasonic flaw detection (USFD), and defective rails are replaced or reprofiled by rail grinding.