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Explanation: The first two cars shown on the first reference web site have the same structure described on the German site that you have mentioned, i.e., a support structure for the load (boiler in your text sample, steam generator in these photos) that provides the necessary stiffness in connecting the truck assemblies (actually flat cars in the reference examples).
An alternative term might be "loading platform", see the second reference. The owner's website on this car is terse but still worth checking: http://emmertintl.com/sitetools/product.php?view=detail&id=1...