I am assuming that the reference is to the Boeing 737 fuselages that move by rail from Wichita KS to Renton WA for final assembly. They move on special equipment; sometimes at the head end of a KCKSEA train and often as a dedicated move, known as the "Boeing special". Of course, no wings, tail, and part of the nose is missing. They are wrapped in plastic, giving them a greenish tint. Here is a link to a post by "ATSF 126" on RailroadForums.Com:
A Week on the MRL
This article contained this excellent picture: