How did Franklin Pierce's child die?

Benjamin Pierce, the youngest was killed while riding on a train. He was riding in a car with parents are the end of a train, in January of 1853, the year that Franklin was set to become President in March. The car became detached from the train, rolled down a hill, jumped off the track and overturned. The train car upset and something hit and killed Benjy. He was the only one seriously hurt.

Frankln Robert died of typhus at age four.

Franklin, Jr, died in three days if birth of unknown causes.