In the Democratic convention of 1852 a dark horse candidate named Franklin Pierce was nominated. He went on to easily win the presidency. Most people don't know anything about President Pierce, but historians rank him as the absolute worst American president of all time. Many think that his actions in office lead to the American Civil War. The two most notable incidents of his term were the Ostend Manifesto (where it was discovered that he wanted to force Cuba into the Union - as a slave state. This angered the North), and the Kansas-Nabraska Act (where the Missouri Compromise was repealed and "bloody Kansas" came into being. This angered everybody).
But . . .
Two months before he was inaugurated he and his family were in a train accident and his eleven old son was killed. So the POD is that when the train derails the son lives and it is Franklin Pierce who dies instead.
Some characters:
William R. King: The Vice-President elect. He is traveling to Cuba in the hope that the environment there will offer some relief to his advanced tuberculosis. In OTL he only served as VP for 45 days before dying.
James Buchanan: Is a political heavy weight who was president after Pierce in OTL (and is listed as the second worst American president of all time). He is also the very close friend and housemate of William King. And he is one of the principle people responsible for the Ostend Manifesto (see above).
Stephen Douglas: Another big name for the Democrats. Senator for Illinois. He ran against Lincoln in OTL. He is also the guy responsible for the Kansas debacle.
Lewis Cass. Yet another top Democrat, he received more votes than Pierce did in the 1852 Democratic convention but did not win the nomination. He is a war hero several times over, he wants to obliterate the Indians, but he is also known as a Great Moderator. Although he was accused of pandering to Southern Democrats on the issue of slavery he resigned as Secretary of State in OTL because he thought Buchanan wasn't doing enough to prevent succession (in a hard-liner way: reinforce Ft Sumter, stuff like that).
---
So it's January 6th, 1853. A train has derailed and the president elect has been killed. He will be mourned by his son, his widow, and by his nation. What happens next? Who will be the 14th President? Who will be the 15th? Will the Ostend and Kansas problems still happen? Will the American Civil War still happen?