A transport container system includes a stackable crate having a bottom element and four side wall elements which are of a dimensionally and pressure stable structure. Each of the side wall elements has a foldable attachment element connected to it. When folded up, the attachment elements of the four side wall elements will form an attachment which will increase the volumetric capacity of the crates. The attachment elements will each bear on an upper side of the respective side wall element and be retained in the folded-up position by guides provided on the side wall elements. When folded down, the respective attachment elements can be integrated into the respective side wall element in such a manner that the attachment elements at least will not protrude substantially over the thickness of the side wall elements.