To solve the question about which example represents uniform circular motion, we need to understand the concept of uniform circular motion:

Uniform Circular Motion: It is the motion of an object traveling at a constant speed along a circular path. Even though the speed is constant, the direction of the velocity vector changes continuously, meaning the object is accelerating due to a change in direction. This acceleration is directed towards the center of the circle and is called centripetal acceleration.

Let's evaluate each option:

1. A train moving on its track: Typically, a train does not move along a circular path in a uniform speed. It moves on a straight or curved path but not continuously circular. Hence, it is not an example of uniform circular motion.
2. A car accelerating on its way: This describes linear acceleration, not circular, and does not imply the path is circular. So, it is not an example of uniform circular motion.
3. A ball thrown vertically upwards: The motion is linear, moving up and then down, and does not involve any circular path. Therefore, it is not an example of uniform circular motion.
4. A fan blade rotating at constant speed: The fan blade is moving along a circular path at constant speed. This is a classic example of uniform circular motion where the direction of motion changes continuously, but the speed remains constant.

Based on the above analysis, the correct answer is: A fan blade rotating at constant speed. This option satisfies the conditions for uniform circular motion as it involves a constant speed along a circular path with changing velocity direction.