Ubi Caritas et amor Deus tibi est.
As far as the train passengers are aware they are sitting still and watching the outside world speed by.
Imagine that the train is moving at a constant velocity of 40 ms-1
A passenger can get up and walk down the train with a velocity of 5 ms-1.
What would be the velocity of the passenger as measured from a stationary person standing watching the train go by?
The train has a velocity of 40 ms-1.
The passengers are sitting or standing still. According to the bystander on the platform they have a velocity of 40 ms-1 relative to him (and the platform)
The passenger has a velocity of 5 ms-1 relative to the train.
The bystander measures the speed of the passenger as 45 ms-1 relative to the platform.
Imagine a person in a space ship which is travelling at 0.8 times the speed of light.
Albert sends a light pulse which travels to the front of the space ship with a speed c.
According to Albert Einstein the stationary observer on the platform would measure the speed of the light pulse to be the speed of light c.
NOT 1.8c. Albert`s idea is that the speed of light is the same in all inertial reference frames!
In the inertial frame of reference of the spaceship travelling at 0.8 times the speed of light Albert measures the speed of the light pulse to be c.
James measures the speed of the the light pulse to be c as well.
In order to explain this idea Albert Einstein wrote his famous Special Theory of Relativity.
The Special Theory of Relativity is called special because it deals only with reference frames that are moving relative to each other at constant speed.
That is spaceships that are moving with constant speed relative to each other.
In each inertial reference frame the Laws of Physics are the same and the speed of light is always measured to be the same.
Inertial reference frame