Railway Engineering — Permanent Way, Rails, Sleepers & Ballast

Railways in India

• The first passenger train in India ran on 16 April 1853 from Bori Bunder (Mumbai) to Thane, a distance of about 34 km.
• Indian Railways is one of the world's largest railway networks under a single management, organised into zones and divisions.
• Advantages of railways: economical for bulk goods and long distances, high carrying capacity, energy efficient, safer, less polluting (especially when electrified), promotes national integration. Limitations: fixed routes, high initial cost, not door-to-door.

Gauges

Gauge is the clear minimum distance between the inner faces of the running rails.

Project Unigauge (launched 1992) converts MG and NG lines to BG for a uniform network.

Factors in choosing gauge: volume and nature of traffic, speed, cost of construction, development of the area, physical features (hilly terrain favours narrower gauges), and uniformity with the existing network.

Permanent way

The permanent way (track) consists of rails, sleepers, fastenings, ballast and formation (subgrade). It is called "permanent" to distinguish it from temporary tracks used during construction.

Requirements of a good track

Correct gauge; proper alignment (straight, curves with transitions); proper cant on curves; smooth and uniform gradients; adequate elasticity and resilience; strong enough for loads and speeds; good drainage; easy maintenance and replacement of components; low initial and maintenance cost; resistance to creep and lateral forces.

Coning of wheels and tilting of rails

• Coning of wheels: wheel treads are given a slope of 1 in 20 (outward taper). On straight track this keeps the wheel set centred; on curves, the outer wheel rides on a larger diameter and the inner on a smaller one, helping the axle negotiate the curve and reducing slip and wear.
• Tilting of rails: rails are placed with an inward tilt of 1 in 20 (by adzing of wooden sleepers or by canted bearing plates/sleeper seats) so that the wheel load acts near the centre of the rail head, reducing wear of the rail head edge and of the wheel flange.

Rails

Functions

Provide a continuous, smooth, level surface for wheels; guide the wheel flanges; transmit loads to sleepers; resist lateral forces and bending; act as electrical conductors for signalling track circuits and return current in electrified sections.

Types of rail section

Weight and length

• Rails are designated by weight per metre; heavy main lines use 52 kg/m and 60 kg/m (UIC 60) rails.
• Standard rail lengths: 13 m for BG and 12 m for MG (fish-plated track); longer lengths are welded.

Wear of rails

On straight track: wear of the top of rail head and at the rail ends (battered ends at joints). On curves: side wear of the head of the outer rail (flange contact) and top wear of the inner rail. Also at points and crossings, in braking/acceleration zones, near stations and on gradients. Remedies: lubrication of outer rails on curves, interchange of rails, use of harder (head-hardened) rails, proper cant, grinding.

Creep of rails

Creep is the longitudinal movement of rails relative to sleepers in the direction of traffic (or downhill).

• Causes (theories): wave theory (wave motion of rail under moving loads pushes it forward), percussion theory (impact of wheels at rail ends), drag theory (backward thrust of driving wheels on the rail, forward drag of other wheels); also braking and accelerating forces, temperature variations, poor fastenings, loose sleepers, steep gradients.
• Effects: sleepers go out of square, gauge and alignment disturbed, joint gaps close or open (buckling or broken fish bolts), points and crossings disturbed.
• Prevention: tightening fastenings, creep anchors (anti-creepers), elastic fastenings (Pandrol/ERC clips which resist creep), adequate ballast and sleeper density, pulling back the rails to their original position.

Rail joints and welding

• Fish-plated joints — fish plates and fish bolts join rail ends; weakest part of the track. Types by support: supported joints (on a sleeper), suspended joints (between two sleepers — common), bridge joints. Arrangement: square joints (both rails at the same point) or staggered joints (on curves).
• Expansion gap at joints allows for thermal expansion: $\delta =L\alpha t$.
• Welding of rails eliminates joints, giving a smoother ride and less maintenance:
  • Flash butt welding (in depots, high quality),
  • Thermit (alumino-thermic) welding (in the field),
  • Gas pressure welding, electric arc welding.
• Short welded rails (SWR) — a few rail lengths welded together (e.g. 3 rails = 39 m on BG).
• Long welded rails (LWR) / continuous welded rails (CWR) — the central portion cannot expand; thermal forces are resisted by sleepers, ballast and elastic fastenings; ends have switch expansion joints (SEJs); must be laid and destressed within a specified rail temperature range to avoid buckling in summer and fractures in winter.

Rail fastenings

Fish plates and bolts; spikes (dog spikes, screw spikes, round spikes); chairs (for bull-headed rails); bearing plates; keys; elastic fastenings — elastic rail clips (ERC, Pandrol type) with grooved rubber sole plates (GRSP) and liners on concrete sleepers — standard on Indian Railways now.

Rail defects and failures

Transverse fissures, crushed or split heads, piped rails, battered ends, corrugation (short-wave undulations of rail head), wheel burns (from slipping wheels), bent or kinked rails, cracks at bolt holes, weld failures. Detected by visual inspection and ultrasonic flaw detection (USFD).

Sleepers

Functions

Hold rails at the correct gauge and inclination; transfer and distribute loads from rails to ballast; provide elasticity and stability to track; maintain alignment and level; resist lateral and longitudinal (creep) forces; (in some cases) provide electrical insulation between rails.

Types of sleepers

PSC monoblock sleepers are now the standard on Indian Railways main lines.

Sleeper density

Sleeper density is the number of sleepers per rail length, expressed as (n + x), where n = rail length in metres and $x$ is a number depending on traffic (e.g. M + 7). For PSC sleepers on main lines it is also stated as sleepers per kilometre (commonly about 1540–1660 per km, corresponding to spacing of about 65–60 cm). Higher density is used for heavy traffic, LWR, high speeds and curves.

Ballast

Functions

Transfers loads from sleepers to formation over a larger area; holds sleepers in position against lateral and longitudinal movement; provides elasticity and resilience; provides drainage; allows adjustment of track level and alignment by packing; prevents vegetation growth.

Materials

Broken stone (hard crushed rock such as granite, basalt, quartzite — best and standard for main lines), gravel, sand, moorum, kankar, brick ballast, cinders/ash (used in the past in yards and new formations).

Size and depth

• Broken stone ballast for BG main lines is angular and graded within specified limits (nominal size of the order of 50–65 mm).
• The ballast cushion (depth below the bottom of sleeper) on main lines is commonly about 250–350 mm, depending on the importance of the route, sleeper type and LWR.
• For effective load distribution, the minimum depth is sometimes estimated as $D=\frac{S-W}{2}$ ($S$ = sleeper spacing, $W$ = sleeper width), assuming 45° load dispersion.

Formation (subgrade)

• The prepared ground surface on which ballast rests — in embankment or cutting.
• Top of formation is given a cross slope (about 1 in 40) away from the centre for drainage.
• Width depends on gauge, single/double line, ballast profile and cess (side space).
• Blanket layer of coarse granular material is provided over weak/clayey formations to prevent mud pumping and improve bearing capacity.
• Formation failures: slips of embankment slopes, heaving/swelling of expansive soils, settlement, ballast pockets, mud pumping — remedied by drainage, blanketing, sand piles, geosynthetics, slope protection.

Resistance to traction and hauling capacity

Hauling capacity of a locomotive $=\mu W_{d}n$ — $\mu$ = coefficient of adhesion (about 0.2 for dry rails, lower for wet or greasy rails); $W_d$ = load on each driving axle; $n$ = number of driving axles.

Tractive effort of the locomotive must exceed the total resistance for the train to be hauled.

Gradients

Grade compensation on curves: gradients on curves are reduced to offset curve resistance — 0.04% per degree of curve on BG, 0.03% on MG and 0.02% on NG.

Curves

Cant (superelevation)

Equilibrium speed is chosen by considering the maximum sectional speed and the average (booked) speed of trains so that cant excess and deficiency stay within limits.

Negative cant occurs on turnouts taking off from the outer side of a curve, requiring speed restrictions.

Transition curves

Railway transitions (cubic parabola/clothoid) introduce cant and curvature gradually.