Last reviewed 30 Sept 2026 · 7 min read
Alignment — what it means
The alignment of a railway is its position in plan (the horizontal alignment: straights and curves) and in elevation (the vertical alignment or profile: gradients and vertical curves). Because a railway is built for a very long life, carries heavy trains with little tolerance for steep gradients or sharp curves, and is costly to change, its alignment is chosen with much care. The plan and profile together must give a line that is safe, economical to build, cheap to operate and maintain, and capable of carrying the expected traffic at the desired speed.
Factors governing the choice of alignment
- Traffic potential — the cities, industries, mines, ports and agricultural areas to be served; the volume and type of traffic (freight, passenger, mixed) and its growth.
- Topography — plains, hills, river crossings and valleys control gradients, curves, cuttings, embankments, bridges and tunnels.
- Geology and soil — stability of slopes, foundation strata for bridges and embankments, and availability of construction materials.
- Hydrology — flood levels, drainage crossings, waterway, and the number and length of bridges.
- Gradient and curvature standards for the class of line and the type of locomotive.
- Land — cost of acquisition, resettlement, forests and environmentally sensitive areas, and existing structures.
- Connection with the existing network — junction locations, station sites and future extension.
- Strategic and safety considerations — border areas, avoidance of landslide, avalanche and flood-prone zones.
- Total cost — capital cost of construction plus the cost of operation and maintenance over the life of the line (a shorter route with steep gradients may cost more to run).
Survey stages for a new line
Selection is by a sequence of surveys with increasing detail, each narrowing the choice:
- Reconnaissance survey — a rapid study of a wide corridor using maps, satellite imagery, aerial photographs and GIS, with a quick ground check, to identify feasible routes and their main obstacles. Several alternative alignments are prepared with approximate lengths, gradients and costs.
- Preliminary (traffic) survey — a detailed survey of the promising routes, with contour maps (often from LiDAR or drones), soil and hydrological data, and traffic projections; estimates of cost and of earnings, and a comparison of alternatives leading to the choice of one route.
- Final location survey — the chosen route is fixed on the ground and set out: the centre line is pegged, longitudinal and cross sections are taken, and detailed plan and profile drawings are prepared with the exact positions of curves, gradients, bridges, stations and level crossings, with the land plan for acquisition and the detailed estimate.
- Engineering-cum-traffic (ECT) survey — carried out in the early stage of the project: it studies the engineering feasibility of the line and its traffic and financial viability, producing a report and the Detailed Project Report (DPR) on which the sanction is based.
Modern surveys use total stations, DGPS, LiDAR (airborne or drone), photogrammetry, satellite data and GIS, which give accurate contours quickly and reduce the number of field visits.
Controlling geometric standards
Gradients
The ruling gradient is the steepest gradient that a train of a fixed load can climb with a single locomotive at the design speed; it decides the maximum load hauled and is therefore a controlling economic parameter. Typical practice on Indian Railways is a ruling gradient of 1 in 150 to 1 in 200 in plains and steeper in hilly country (for example 1 in 100), with pusher or helper gradients (as steep as about 1 in 37 in the Ghats) where a helper locomotive is used. Gradients in station yards are kept as flat as possible (a maximum of the order of 1 in 400) so that standing vehicles do not roll away. (Values are typical; the current standards and the class of line decide.)
Grade compensation on curves reduces the steepness of the gradient to allow for the extra resistance of the curve (see the gradients note).
Curves
Sharp curves increase the resistance to traction, wear of rails and wheels, and limit the speed. The degree of curve is (with in metres). Standards set the maximum degree of curvature permitted for each class of line — the sharpest curves are avoided on main lines (a limit of the order of 10° on BG in exceptional cases), and transition curves are provided at both ends of each circular curve. Curves are separated by adequate straight lengths (or, for reverse curves, a minimum straight) so that the cant can be run off.
Vertical curves
Where the algebraic difference between two gradients exceeds a small value (of the order of 4 mm per metre), a vertical curve (a parabola) is introduced so that the change is gradual.
Other points
- Locate stations on straight, level or nearly level ground with a good site for future expansion and road access.
- Avoid crossing rivers at skew and at places of unstable banks; bridges are placed at the best crossing even at the cost of a longer approach.
- Keep cuttings and embankments balanced (the cut earth fills the embankment) to reduce the leads and cost.
- Cross roads at right angles and preferably grade-separate (ROB/RUB) at busy roads.
- Provide drainage to protect the formation.