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How to Keep your Train from going off the Rails

October 3, 2016.

Thirty years ago, I received a beautiful electric train set.It came with three different kinds of rails:

How to Keep your Train from going off the Rails

Thirty years ago, I received a beautiful electric train set.

It came with three different kinds of rails:

Different types of rails

  1. straight rails,
  2. curved rails,
  3. and… curved rails… what ? Again?!

Years later, this third type of rail still puzzled me. The second kind of rail was definitely arc-shaped, but the third had a funny curve to it.

One thing was sure: when I would build an oval run, if I didn’t put the right rails in the right place, my train was guaranteed to derail at high speed.

Fleischmann Box

All aboard the clothoid!

Enough with the teaser. The mystery curve was called a clothoid, or Euler spiral, or just spiral among friends.

A clothoid is the result of a specific mathematical formula that defines a curve whose curvature changes linearly with its curve length.

By definition, the radius of a curve is:

  • the radius of a circle (in our case, the second type of rail),
  • infinity for straight rails (i.e. the same rails with an infinite radius).

Following the sequence below (called “S-C-S” for Spiral-Curve-Spiral):

  • rail-straight, R = +∞,
  • rail-spiral, R : +∞ → 60 cm,
  • rail-curve, R = 60 cm,
  • rail-spiral, R : 60 cm → +∞,
  • rail-straight, R = +∞,

the train trip becomes much smoother, without the risk of derailments!

To better understand the effect, let’s take another example from railroads:

The blue curve is exclusively made up of arcs from a circle with a 60-cm radius, while the green and red curve is made up of two different spirals.

If we now represent the curve (1/R) along the road, this is what we get:


The second itinerary is longer, but free of curvature breaks. This type of curve has many applications in civil engineering: for roads, bridges, railroads, mechanics.

Remember this next time you take a highway exit safely at 100 km/h!


Why are clothoids also called spirals?

Looking at the math behind the curve, if you draw such a curve on an infinite trajectory, this is what you get:

Two symmetrical spirals.

In the rest of this article, we’ll use the following symbols:

R Radius of curvature at any given point in a trajectory.
Given that R = +∞ at the beginning of the trajectory.
φ Angle between the tangent to the trajectory at any given point and the axis of the abscissa.
φ = 0 at the beginning.
L Distance travelled along the curve at any given point.
L = 0 at the beginning.
A Constant describing the change in the radius of the curve along the trajectory, by definition:
1/R = L x 1/A2, i.e. A2 = RL

Note that in this figure, the two φ angles are identical:

  • φ = internal angle between (d1) and (d2).
  • (d3) is perpendicular to (d1) by construction.
  • (d4) is perpendicular to (d2), since it is the tangent of circle CM at point M.
  • Therefore the angle between (d3) and (d4) is also φ.


Let’s draw a clothoid!

Now let’s draw our own spiral.

If we imagine the trajectory of point M(x,y) along the following itinerary:

  • from the curved abscissa L1 (=0),
  • to the curved abscissa L2 (=L length of trajectory),

we get:

Note that φ is indeed a function of l:

So, given:

we get:

which gives us the following JavaScript code:

function drawSpiral(A, L1, L2)
    var a  = A*Math.sqrt(2);
    var S1 = L1/a;
    var S2 = L2/a;

    var s=S1;
    var ds = Math.abs(S2-S1)/10000;

    var M = new Point(0,0);
    while (s



  • Length :
  • StartRadius :
  • isCCW:
  • isEntry:
  • ClothoideConstant: