I just checked my David Doyle, and McComas & Touhy, plus the website Postwarlionel.com and none of the reference the 2857 Caboose yet clearly it exists both in the 46 Catalog and actual rolling stock.
I have no Greenbergs, does he make mention of them?
Further research shows while the 2857 was pictured in the catalog, it was never produced, the Berkshire set delivered with a 2457 PRR tin caboose. It would follow that the ones shown so far are pre war 2957s with post war trucks.
Pete