The Mystery of the Hidden Painting

Henry, Jessie, Violet, and Benny used to live alone in a boxcar. Now they have a home with their grandfather, and they find a mysterious painting hidden in the attic.
The children learn that it is a portrait of their grandmother, whom they never knew, wearing the necklace Grandfather gave her when they were married. The necklace disappeared on their first anniversary - and was never found. Can the Boxcar Children track down a necklace missing for so many years?