Driving a railway wagon through a curve
Maybe you noticed that some railway wagons have fixed axles between wheels. Left and corresponding right wheel on a wagon are connected by a rigid axle. This way it is much cheaper to suspend the wagon, but it also forces both wheels to rotate at the same speed.
How then a railway wagon makes a turn? While traveling across a curve the outside wheel should travel a greater distance than the inside wheel. But paired railway wheels cannot turn at different ratio!
The explanation is simple and fascinating. Look at the picture below. The upper (red-crossed) part of picture shows how train wheels DO NOT look like. Only the lower part of picture shows the real stuff.
Notice that train wheels are in fact cone-shaped. Also the top of the rail is not totally horizontal but is inclined. When traveling in a straight line this configuration helps guiding the wagon by forcing it to the center.
Even more interesting thing happens while traveling through curved section. When in curve, railway wagon is forced a bit to outside by centrifugal force. This forces the outside wheel to a bit larger diameter and the inside wheel to a bit smaller diameter. Both wheels are still turning at the same speed, but the outside one travels a greater distance just as it should be.
I was amazed when I first learned all this. Hardly it can be simpler than that.
Of course, there are disadvantages. First, weight of the train pushes rails aside (not only vertically). This is not such a big problem because you have to fix rails firmly and securely anyway. Second, rail curvature must be anticipated for an exact speed. This is also not a big problem because rails are actually used that way.
Danijel Gorupec, 2006