How did Ponyboy's parents die in The Outsiders?

Ponyboy never elaborates about the exact cause of death of
his parents in Susan E. Hinton's classic teen novel, The Outsiders.
We are only told that the three brothers had been orphaned when their parents died in an
automobile accident. In the movie version of The Outsiders,
director Francis Ford Coppola used a dream sequence of the parents dying in their car
when it was hit by a train, but the novel does not include the details about the train.
We do know that Pony's dad was named Darrel (Darry was a Jr.) and that the mother was
very beautiful.