By Anupum Pant
I’m pretty sure not many of you know this about train wheels, neither did I.
Look at the picture and answer this: What do you think keeps a train moving on the track? or Which part of the wheel do you think it is that keeps the train from careening away from the track at turns?
Applying general logic, I thought that flanges at the end of the wheels kept a train from going off rails at a turn. Turns out, I was wrong!
In fact, flanges at the end of the wheels are just a safety mechanism to keep the train on its track only if the main mechanism fails. And what is that main mechanism?
Train wheels are conical in shape. That means they have a varying diameter at different points of contact. Now, suppose the track turns right. The train’s left wheels now have to travel more than the right wheels because at the turn the track on the left is longer.
So how do the left wheels travels more than the right wheels without a differential?
Since the wheels are conical in shape, the whole wheel-set shifts a bit to the left, if the track curves right. Now the point of contact of the left wheel is at a larger diameter of the cone. While the smaller wheel 
touches at a point where the diameter of the wheel is lesser. Therefore, if the left wheel now makes one circle it travels further than the right wheels and the train moves along the curve smoothly.
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.
Look at how you can try this at home using 2 plastic cups and 2 similar pipes. [Experiment]
If I couldn’t explain it properly, probably the best physics teacher ever – Richard Feynman – will explain it to you better. [Video]
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