The EMD GP9 is a Four Axle General purpose diesel locomotive produced in America by the Electro-Motive Devision of General Motors (known as Electro-Motive Diesel today).
While the EMD GP7 proceded the GP9, the GP9 was part of the diesel revolution that took over railroads in the 1950's considering that diesel locomotives like the GP9 (and its bigger brother the SD9 which is not included in game) where more flexible and cheaper to run then steam engines.
Like the EMD SD9, the GP9 still serves major and minor railroads today, some in its original high hood forms, while some railroads have modified their GP9's or ordered them with a low short hood like more modern locomotives. Many railroads have also upgraded their GP9's over the years producing whats called a GP9M or U depending on the railroad, and tend to have new or recylced engines and electrical and in some cases even the trucks are replaced to bring the GP9's up to modern standards.