Question in understanding the special theory of relativity

Hello Rabbi Michael,
I’m reading your book Freedom Comes and on page 145 you talk about Einstein’s special theory of relativity and I just can’t understand.
The example is that a train traveling at the speed of light and sending a beam of light upward at the speed of light at point A will arrive at point B at the same time, from the perspective of the person sitting in the train, if there is a mirror, etc. A few questions:
1. You wrote that Einstein and his friends claim that the speed of light maintains a constant speed of light – what causes it to move diagonally if so? Doesn’t there need to be a force exerted on the light to cause it to move diagonally?
2. Even if a force is applied, and for some reason the light maintains the speed of light even though there should have been acceleration here, how is it logically possible that they arrive at the same time? After all, the light traveled a greater distance?
3. And if it is a fact that this is how it is, how did Einstein arrive at this in thought? What logical operation is there here? Is there thinking against logic here, and what then is the thought experiment here? If he had empirical information that this is how it happens, I do not understand the greatness of the thought experiment

thanks,