Now, without the separation, the two families in the car were one. The men sat together, and their spirits were damp. Ma kept a little fire going in the stove, kept a few twigs burning, and she conserved her wood. The rain poured down on the nearly flat roof of the boxcar.
– John Steinbeck
The Grapes of Wrath, Chapter 30. Al Joad takes the tarpaulin down from the middle of the Joads’ and Wainwrights’ boxcar, to use to protect the truck from the rains. This symbolically joins the two families as one.