How to detect bend in a spline or strait part of spline at position

[velikizlivuk]

I used 2 variables for storing tangents during Update method

private Vector3 previousTangent=Vector3.zero;
private Vector3 currentTangent=Vector3.zero;

and in Update:
// Check if we have current tangent and set as previous
if (currentTangent!=Vector3.zero)
{
previousTangent = currentTangent;
}
// Angle on Z axis to lean object on rail
sideLean = (Vector3.Angle(currentTangent - previousTangent, transform.forward) - 90) * (Speed/MaxSpeed)*MaxLeanFactor;
// Apply lean angle
this.gameObject.transform.localRotation = Quaternion.Euler(0, 0, sideLean);

Speed is current speed of object on rail
MaxSpeed is maximum speed on rail
MaxLeanFactor is factor for making lean more or less

I'm not sure if that's working correctly since I discarded that feature.