EMD GP35 no. 2500 (builder no. 29595) was acquired new in December 1964.  In January 1965 it was renumber to 2501.  Why in the heck would they simply renumber it a year later?  I asked this question to a long time employee of the Alaska Railroad and was told, "The story I got was that back in the days when the train number was diplayed in the number box (e.g. X2500) the roundhouse kept running out of zeros.  The 3000 was also renumbered (to 3006) at that time. It seems to me buying more zeros would have been easier, but that was the story.  The scheme we use now (lately for cars as well) makes sense to me.   Under it, 2500 designates the class, and each unit in the class has its sequential number, 1 to whatever."