From the back of the book:
Henry, Jessie, Violet, and Benny used to live alone in a boxcar. Now they have a home with their grandfather and are spending the summer in an enormous Mew England farmhouse.
There's an old woodshed near the house that holds a surprising secret - a secret that dates from the Revolutionary War. The Boxcar Children don't know it yet, but they are about to uncover The Woodshed Mystery!