Why Railway Curves Aren’t Perfect—and Why That’s on Purpose
- thepwayengineer
- May 25
- 2 min read
Why Are Trains Not Balanced in Curves?
Most people assume a train should be perfectly balanced on a curve—tilted just right, riding equally on both rails. But that only works for one train, at one speed. On real railways, things aren’t that simple.
Enter cant deficiency.
What Is Cant Deficiency?
Cant deficiency is the difference between the actual cant applied to a curve and the theoretical value (equilibrium cant) that would perfectly balance the forces on a train at a specific speed.
Because trains don’t all run at the same speed—especially on shared lines—engineers intentionally design curves with less than the ideal cant. This lets faster and slower trains both use the same track safely.
The Physics Behind the Design
When a train enters a curve, it accelerates laterally. Combine that with gravity, and you get a resultant force that pushes outward from the curve.
To manage this, the outer rail is raised (that’s the cant). When the cant is just right for a given speed, the resultant force acts directly downward—this is equilibrium cant.
But that’s only perfect for one speed.
Why Engineers Use Cant Deficiency
On mixed traffic lines, applying full equilibrium cant would over-cant the curve for slower trains, increasing risk and discomfort. Cant deficiency lets designers create a safe, functional compromise.
Even on passenger-only routes, cant deficiency can improve bogie steering, reduce hunting, lower rail wear, and prolong component life.
Limits and Considerations
Too much cant deficiency can overload the outer rail, increase wear, and cause issues like wheel unloading or poor suspension behaviour—especially on rough track or in extreme temperatures.
That’s why rail authorities limit cant deficiency values, and why track designers must weigh traffic type, infrastructure condition, and geometry constraints before deciding how much to apply.
How Tilting Trains Extend the Limits
Tilting trains don’t reduce the force on the rail—but they reduce the lateral acceleration felt by passengers. That’s why they’re used in combination with higher cant deficiency on certain intercity routes.