The name "u-boat" for any of the GE U series locomotives was mostly, if not entirely, a railfan thing. The Rock Island operated a number of different models in the U series, and I never heard a RI employee, or an employee of any other railroad who was not first a rail fan, use the term. During WWII the numbers of German Submarines assigned by the German navy all began with a prefix "U", e.g., the U-505 at the Museum of Science and Industry in Chicago. The U was an abbreviation for the German word for submarine (Unterseeboot) and most model railroaders in the 1960's and 1970's were familiar with naming convention for German submarines, so it was something of a natural, if dated, railfan moniker.