Thanks, Mike. One more question. In the schematics I see a hinge at the midpoint of the articulated frame. From which end of the locomotive did one truck swing, when rounding a curve? Or was there some sort of pivot at the mid-point of each truck?
I don't know the details about the GG1's running gear design, but I think the main purpose of the 'hinge at the midpoint of the articulated frame' is to transmit longitudinal forces between the two trucks (I mean the frames with the 3 powered axles each). This is of advantage to the main frame as the couplers are mounted on the trucks. In case of slack running in or out the forces were transmitted directly from one truck to the other so the main fram has not to bear these forces.
The hinge can have a second purpose : to join the two trucks vertically (transmitting vertical forces). In doing so, the load transfer (change of axle loads) due to traction/braking efforts of the whole loco is kept very low. Without this joint the front end of a truck slightly moves upward and the rear end downward when the truck exerts tractive effort. Linking the inner ends of both trucks reduces the pitch movements of both trucks resulting in a minimum amount of laod transfer. A minimum of load transfer was desirable as the axles could gain slip independently from each other under reduced wheel/rail adhesion.
The hinge could even have a third purpose : to join the two trucks laterally. This feature is used to reduce the lateral wheel to rail forces when running through a curve. To understand this, you should first realize that a truck always would like to go straight ahead. Going through a curve means the truck has to turn around a vertical axis. This turning movement requires some sliding of each wheel against the rails. To turn a truck through a curve, a lateral force is required to overcome the wheel/rail friction because of the sliding movements. In most curves the outer wheel of the leading axle touches the rail head with its flange thereby getting the lateral force from the outer rail in the direction to turn the truck. In tighter curves the inner wheel of the trailing axle will also touch the railhead (of the inner rail) with its flange, thus getting a lateral force for turning the truck, too.
When two trucks are running independently from each other under a common main frame, one will find that (when travelling through a curve) they swing out under different angles in respect to the main frame, the leading truck having a smaller and the trailing truck having a larger angle. This is the reason for the hinge points of the inner ends of the truck frames not remaining face to face when running through a curve. When travelling through a lefthand curve the hinge point of the leading truck has moved to the left in respect to the hinge point of the trailing truck. Now consider the two hinge points connected to each other : this means the leading truck has to increase its angle of turn against the frame and the trailing truck has to reduce its angle. In doing so, the alignment of both trucks in respect to the curve has improved and the lateral forces at the wheel flanges are lower than without joining.
If the lateral joint is strict and stiff, it will lead to severe hunting of both trucks when running on tangent track at high speeds. Therefore, this joint is normally designed with a specific amount of play. But even if the play has been exhausted the joint is not made by a stiff link but by means of a pre-stressed spring.
Whether the linkage in case of the GG1 has which of these 3 options you can perhaps only decide by knowing more details, e.g. construction drawings of the linkage.
The design of such a linkage between the truck frames is very much "steam era thinking" - it was very much common practice at that time to put a considerable amount of effort into a good curving behaviour and low load transfer.
The main frame of the GG1 seems to me to rest on four spring loaded plates which are located in such a way to get the desired load distribution on the drivers and on the undriven axles. Longitudinal and lateral guidance could have been by two vertical pivots on the main frame, each fitting into a hole in the truck frames (location seems to be about midpoint between the second undriven and the first driven axle). It is possible, that these pivots do not transmit vertical loads (as a 'center plate design" would do), so that the main frame completely rests on springs - this would lead to a much smoother vertical ride (which has been reported).
In the era of the 20s and 30s a number of electric locos (having two trucks) had a linkage between their trucks. Today, the trucks are arranged at a much larger distance between them, and tanks on diesel locos and transformers on electric locos fill the space. Today, a linkage between trucks isn't a running gear design feature anymore. Instead, in some cases radial steering trucks are used to reduce the wheel flange forces and wear.