Answer:
The best answer to the question: Which theory of relativity explains the following scenario:____, would be: The General Theory of Special Relativity.
Step-by-step explanation:
The General Theory of Special Relativity, also known as Special Relativity is a theory that was proposed by Albert Einstein and it was published to the public in 1905. This theory is today the theory of relativity that is universally accepted and has been already proven by experimentation, and therefore confirmed. In the case of the statement of this little boy, this theory would apply because it proposes that time and space would be linked for a objects that are moving at a constant and similar speed and in a straigth line and therefore it would seem as if he, relative to the two moving trains, at constant speed, and in a linear fashion, were not moving at all.