Conversation so far:
who did Edwin Booth save?
Abraham Lincoln's son, Robert, from serious injury or even death.
where did this happen?
The incident occurred on a train platform in Jersey City, New Jersey.
What caused this?
The incident occurred while a group of passengers were late at night purchasing their sleeping car places.
What happened next?
The platform was about the height of the car floor, and there was of course a narrow space between the platform and the car body.

Question: Why did he fall?
Self-contained rewrite: Why did Robert Lincoln fall?