Note: pages still being revised
The GM Model Information Page

GM models have followed a set pattern over the years. Some of the oddities come from language translations (KK16 and TT12 are from German)

Export models are to suit local conditions, and are named from a standard EMD convention of letters/numbers:

1st letter indicates:
A= cab unit design (eg. A16C)
G= standard roadswitcher design (eg. G8)
J= double ended, end cab design (eg. J12)

2nd letter indicates:
A= electro-mechanical underslung body mounted t/m's (eg. GA8) except when used with cab units, indicates a second cab (eg. AA12).
L= lightweight frame (eg. GL8, GL22CU)
M= special modification (eg. GM6)
R= obsolete designation for 6 axel heavy duty r/s design (eg. GR12)
T= turbocharging (eg. GT16)
Numbers indicate engine used:
6= 6 cyliner 567 engine
8= 8 cylinder 567 engine
12= 12 cylinder 567 engine
16= 16 cylinder 567 engine.
. add 10 to the number to get the 645E engine
. add 20 to get the F engine subseries
. add 30 to get the 710G engine.
. (exceptions are the 6 cylinder 645 engine stays as 6... not progressed to 16, which was already taken)

Letters after numbers indicate:
L= lightweight frame for turbocharged (eg. GT22LC-2)
C= 3 axel truck(s) all axels powered (Co-Co). Not used on early exports.
A or 6 = 3 axel 2 t/m truck design (A1A-A1A) (eg. G12A, GL8U6).

No letter is used for 2 axel/t/m design (Bo-Bo).
U= narrow gauge t/m's (eg. GT22CU)
W= standard gauge t/m's (eg: GT26CW)
-2= Dash 2 design components and electronics in 645E engined locomotives.
There are exceptions to standard designation rules (eg. G16 and G16C are the same thing)