If you drop an object through the window of the fast moving train, it will follow a curved path and hit the ground. Because, the object carries train's speed even outside the train and gradually slows down. If you fire a gun from fast moving vehicle, you have to add vehicle speed to the bullet's speed.
Suppose, an express train's head light is switched on. The light coming from the lamp travels with its own speed, that is the speed of light. You need not add the train's speed however fast the train may be.
Light is an electromagnetic wave. Here "changing electric field is created by the changing magnetic field and vice versa". Using this mechanism only, light wave progresses. Hence the light wave propagation is independent of its source's motion.
This electromagnetic process cannot be influenced by any other physical phenomenon. So the light wave cannot be sped up (pushed) or slowed down.(pulled)
The formula for the sped of the light is
Here all are constants
Hence the speed of the light is also absolute universal constant.
Unlike other motions, the light ray does not slow down and come to a halt. It travels for ever with fixed velocity. This concept is discovered by maxwell purely using mathematics and it is the corner stone Einstein's theory of relativity.
Note: Suppose light travel through a slab of glass, it will be slowed down by the glass. But once the light enters the air. it gets back the original speed.