When and how did Pony's parents die in The Outsiders?

This question is never fully addressed in S. E. Hinton's
novel, The Outsiders. Pony reveals that his parents were killed in
a car wreck, but there are no other specific details. We know that Pony's father
(apparently Darrel Curtis Sr., since Darry is his namesake) was 40 years old when he
died and Darry bears a striking resemblance to his dad who, according to Pony, looked
only 25. In the Francis Ford Coppola film, the director shows Pony reliving his parents'
death in a dream sequence showing them struck by a train while in their car. However,
the train is not mentioned as a cause of death in the novel.