ModelRailroadForums.com is a free Model Railroad Discussion Forum and photo gallery. We cover all scales and sizes of model railroads. Online since 2002, it's one of the oldest and largest model railroad forums on the web. Whether you're a master model railroader or just getting started, you'll find something of interest here.
The GP7, GP9, GP18 & GP20 all share the same chassis (although with almost all of the GP20s you would have to change the fuel tank*). The GP30 chassis has a longer bolster spacing.
*Great Northern's GP20s had the same fuel tank and air tank arrangement as the GP9/18