After having been succeeded by the more modern SD40-2, a large number of SD40 have been upgraded with more modern equipment. The main difference between rebuilt SD40 and genuine SD40-2 is that the latter use a longer frame and has longer front and rear porches, while the rebuilt SD40 have a shorter frame. A common product of rebuilding SD40s, and sometimes SD40-2s, is an SD40-3. The major add…