What’s so different about train wheels : Differential mechanism in trains

Have you ever wondered what keeps the train on the track? When a track takes a turn what makes the train stay on the track instead of just continuing in its direction of motion? After all, there is no steering in a train

Many of us would think that the flanges on the wheels would do the job.

But that is not the answer.

The flanges are just a safety device. If the flanges rub against the track, it gives a horrible noise and it is a huge waste of energy. Moreover, there will be wear of tracks and wheels.

The flanges are a secondary mechanism just in case the real mechanism fails  

There is an another problem connected to it: Rail wheel differential design

There is something fundamentally different in the design of differential of a train and an automobile.So, why is it different on a train?

  1.      Unlike a normal locomotive, trains have a huge body
  2.      Trains don’t make sharp turns like an automobile

Hence adding a differential would be a potential waste of resources.

So, how does it go around a corner?The answer lies in the geometry of the wheels and tracks

The wheels are tapered, conical in shape.That means they have a varying diameter at different points of contact


Suppose the track curves right,the whole wheel-set shifts a bit to the left.


The point of contact of the left wheel is at a larger diameter of the cone. While the diameter at the point of contact on the right wheel is much smaller.



As both the wheels are connected by a solid shaft both the wheels must the same angular velocity. Since velocity is a function of diameter and angular velocity, the left wheel has a greater velocity than the right wheel. This forms the differential in a train

The whole beauty of this system is that the amount of shift of the wheel set happens automatically, makes the train move on turns smoothly and keeps the train on track.

Hence the conical geometry along with the flanges ensures the train stays on the track

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