Somebody remarked that streamlining was originally intended to improve efficiency by reducing wind resistance but quickly was adapted to increasing revenue by reducing ticket sales resistance. The first American streamlined steam locomotive was NYC 5344, the Commodore Vanderbilt, called the Flying Bathtub, designed in a wind tunnel at the Case Institute of Technology. It was followed by Henry Dreyfuss' Mercury Pacifics and Century Hudsons, which were purely styling excercises.
Raymond Loewy did Pacifics, the S1, and T1 for the Pennsylvania. Otto Kuhler did the Atlantics and Baltics for the Milwaukee Road and other jobs for various lines plus becoming the Alco stylist for some years. My favorite of his jobs was the Pacific for the Royal Blue.
The SP Daylight streamlining was an in-house job and looks it, as was Burlington's Aeolus (Big Alice the Goon). Who did Santa Fe's Blue Goose I don't know. And there were several other home-brews.
The LNER A4 was styled by Sir Nigel Gresley after looking carefully at a dirigible. Again it was an attempt at reducing wind resistance. As for the French, Belgian, and German streamlined steam, can anyone enlighten us? And indeed there was a Mongolian streamliner....