Here is my understanding of the origin of the term “SP-Type” caboose. When Lionel Corporation created the tooling for this style caboose in ‘47, the first ones (2257 & 2357) came decorated with the graphic “SP”, abbreviation for Southern Pacific. This term has stuck ever since to describe this style caboose with Lionel train enthuses. Here’s an example of the 6357 SP Caboose produced from ’48 to ’61.
Perhaps they were an early user of steel bodies? I can tell you one thing, there are A LOT of surviving Southern Pacific Cabooses of evey shape and description out there! [:D] http://coscia-espee.info/sp_cabooses.html