The Pennsylvania Railroad GG1: Wheel Arrangement
F. M. Whyte's system of classification is used to describe the wheel arrangement of conventional steam locomotives. In this system, a series of numbers separated by hyphens, is used to indicate the quantity, location and use of the wheels. The first number is the number of leading wheels and the last is the number of trailing wheels. The middle number (or numbers) indicates the numbers and arrangement of the drivers. If the GG1 were a steam locomotive, it would be classified as a 4-6-6-4 in the Whyte system.
The system used to describe electric locomotives uses a series of numbers and letters separated by hyphens to indicate the number of powered and non-powered axles. Non-powered axles, each with two wheels, are represented by the number of them. Powered axles, each with two wheels, are represented by letters as follows: A = 1, B = 2, C = 3 and D = 4. Separate frames are indicated with a + sign at the point of articulation. In this system the GG1 is designated as 2-C+C-2.
On the PRR, steam locomotives were classified using single letters starting with "A" for the 0-4-0 and moving down the alphabet as the number of wheels increased. Ten wheelers (4-6-0) were designated as Class G. The GG1 is an articulated locomotive with two separate frames each having a four wheel leading truck and six drivers or the equivalent of two ten wheelers back-to-back, thus the "GG" and the one indicates that this was the first design of locomotive to have the 2-C+C-2 wheel arrangement on the PRR.