EMD SW7 Phase Guide and Builder's List
EMD replaced the 1,000 horsepower NW2 with the 1,200 horsepower SW7 in October, 1949. The SW7 continued to be built until it too was replaced by the SW9 in November, 1950. There was some overlap in production, until January, 1951. This was a popular locomotive, and EMD could not keep up with demand. Southern Pacific wanted to place significant orders but had to settle for just four units for the Cotton Belt because of long lead times. Here's the link to the .pdf