4 Feb 2012 - How trains stay on the tracks

I was talking to a friend of mine today and he was telling me how he may have found himself a new idol, Richard Feynman. I have never heard of him but my friend was explaining that he is a Physicist but is now deceased. He was telling me about these YouTube clips featuring Feynman describing various scientific concepts. One that intrigued my friend, and me when he told me, was how trains stay on their tracks. I had honestly never really thought about the why, I had just taken it for a given that they did. When my friend asked me to think of why, I didn't really have an explanation. So he told me what he had always thought was that it was the lip of the wheel, that sits below the track, that ensures that the train does not deviate from it's set path. This seemed reasonable to me, but after further discussion and watching the YouTube clip I discovered that it is much more involved than that!

Feynman describes the design of the train wheels and how they are tapered so that the outside of the wheel is smaller in diameter than the inside part which is closer to the train body and how both wheels are connected by a steel rod. This wheel design ensures that if a train starts to deviate from the track, the wheels will move so that the deviating wheel it is doing a larger rotation than the opposite wheel, thus pushing the train back into place. To help visualise this, imagine that you have small wheels on one side of a toy car and larger wheels on the other side. When you push the toy car, it will turn to the side of the smaller wheels. This is the same with the train, if it is deviating from the track then it's moving the tapered parts of the wheels so that the side that is deviating is taking a bigger rotation, which then means the other side is taking a smaller rotation. This forces the whole train to  move back towards the smaller rotation and therefore back on track. The lip on the wheel is only a back up for if the tapered wheels fail. This wheel design also assists the train to turn corners. When the track bends in a certain direction the centrifugal force (for an explanation of this, see my 1 Feb 2012 post) pushes the train away from the direction the train is turning, which shifts the outer wheels so that they are doing a larger rotation, thus assissting the train to turn in the direction of the bend.

To watch the clip, check out YouTube.