Track geometry is the shape of a railway track: its horizontal alignment, its vertical profile, the gauge between the rails, and the cant (the height difference between the rails on curves). Designers set it, and maintainers measure and correct it throughout the railway's life, within tolerances of millimetres.
At a glance
| Horizontal | Straights, circular curves and transition curves |
| Vertical | Grades and vertical curves |
| Cant | Outer rail raised on curves (superelevation) |
| Gauge | Distance between the rails; 1,435 mm on standard gauge lines |
| Tolerances | Millimetres for gauge, cant, line and level |
The parameters
| Parameter | What it describes |
|---|---|
| Radius | How tight a curve is |
| Transition length | Where curvature and cant change gradually |
| Cant | Rail height difference on a curve |
| Cant deficiency | How much more cant would balance a faster train |
| Twist | Change of cant over a short length; limited for safety |
| Gauge | Distance between the running edges of the rails |
How it is used on projects
Track designers set the geometry to the line speed and the trains that use it; structures, platforms, overhead line equipment and signalling are designed around it. Clearances are checked against the space the moving train needs (the gauge profile or kinematic envelope), which depends on cant and curvature. In IFC 4.3, alignments carry cant, so models can position elements against the true track geometry.
In operation, track recording measures the geometry regularly and maintenance corrects faults before they exceed limits. Because tamping and renewals can move the track slightly within its allowances, as-built track positions should be resurveyed before new structures or equipment are designed close to it.
Variations
- Gauges: standard (1,435 mm), broad and narrow gauges in various countries.
- Ballasted and slab track, with different construction and maintenance tolerances.
- Switches and crossings, where geometry is more complex.
Common mistakes
- Clash-testing structures against the track centre line, not the kinematic envelope.
- Ignoring cant when checking clearances on curves.
- Mixing alignment revisions between track, structures and OLE teams.
- Treating track geometry as fixed, when maintenance can move it within allowances.
Where to read more
The infrastructure manager's track design standards (for example national railway group standards); the European technical specifications for interoperability for European networks; and the IFC 4.3 documentation for rail alignments and cant.