Interesting detail. I have a fleet of 224 engines and never seen/noticed that detail. Maybe I should check closer. If the trailing truck originally came on the engine, it would have been 1946 as lot of production items, in this case black hand railings and square-backed engines, were left over from 1945 and dropped into 1946. The short drawbar would never fit into a 1945 tender. But that nut/screw looking combination is definitely odd. Like many outliers, unless the exact history is known, it may never be known whether they were part of a Lionel experiment or altered by a do-it-yourselfer for a specific purpose. Without being able to examine it closely, here are some questions/thoughts that might shed some light. I can’t tell if there is a screw head on the bottom or another method to rotate the screw-like protrusion to vary it’s heighth, because that would seem to be its purpose. If the screw is fixed in place, it just might have been the easiest way to attach it to the trailing truck. Check the contact rollers of the engine. Are they original? And how much wear do they have? I’d expect comparable pitting/wear on the bottom of the trailing truck where the screw would make contact. Most likely, it would be shaped in an arc where the point of contact changes as the engine rounds a corner. In unison, I’d carefully the examine the paint on the engine and trailing truck for signs of being repainted, like scrapes in paint that have been painted over. If the addition was done by a hobbyist, a fresh coat of paint may hide the evidence on the trailing truck. Because trailing truck swivels in an arc and the back of this engine is straight, I’m at least somewhat skeptical that it would be useful in keeping the distance between the wheels and the frame during a turn. But that protrusion might be useful when the engine goes straight through switches and crossovers of different manufactures where the wheels might bounce up from different surfaces. And, it might be useful for tender drawbars that have a hole in them to attach to. I think there are several prewar tenders that attached in that fashion. Another here's another detail that might give away where the engine fits into the timeline. Almost all 224s are held together with a screw through the smokestack that utilizes a Phillips head. The very last part of production in 1946 often had a slotted screw instead. I have to admit, I can think of several reasons why this might be added by an operator, and no reason why Lionel might experiment with it, especially with a proven design during the last year of production. . . . but that is hardly definitive. I hope this helps.
Dan