This is slightly off topic, but not entirely. We all are aware of the art-deco streamling craze that swept American railroads during the thirties. Artistic design is not necessarily fuel efficient, so what might be "art", might only be pleasing to the eye. We all know what happened to streamlining once shop time and other issues regarding service were calculated into the equation. I'm curious, if streamlining was ever seriously applied with an eye toward fuel efficiency? Furthermore, how much fuel savings would the application of aerodynamic efficiency to a modern steam locomotive produce?
There has been a great deal of talk about roller bearings on steam locomotives as being an "ideal". Taking a locomotive like N&W 611 or C&O 614, how would serious aerodynamics improve their operation? How much fuel might be saved, if any?

I seem to remember seeing a paper/article or two on this from back in the day saying that fuel economy was actually reduced by a significant margin due to the streamlining adding so much weight to the engine. Unfortunately, I can't find the article(s) now.

For something to think about, I think it somewhat depends on what you're looking for. If you only consider the actual studies done on steam engines in service, you would be excluding modern materials. But, if you're wondering about either modifying an existing steam locomotive or building a new one, it opens up a whole raft of modern light-weight materials that could rather heavily sway the results.

1940's era trains did not operate at high speeds consistently enough to benefit. Today's TGV is another matter. Also, the "low hanging fruit" was weight reduction.

And of ccouse streamlining a steam locomotive ADDS weight.

I remember reading somewhere that the streamlining on cars actually increased the friction on the flanges caused by side winds. The cars had more wind resistance to the sides cause by the additional skirting.