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Dimastep-- I tried to find a link to a top-down drawing of the 3-truck assembly, but was unsuccessful. You're almost there. But the I-beam can also pivot. Think about the front engine of a UP Big Boy or other articulated steam locomotive. It's a 4-8-8-4, where the front cylinders and 8 drivers are called the front engine, and the rear 8 drivers and cylinders are the rear engine. The locomotive is NOT on a single rigid frame--if it were it could never go around many main line curves, it's so long--instead, there are two frames-one rigid and one that swivels, connected by a pin in the middle so the locomotive can "bend". The front set of driving wheels and the lead truck are on a separate, swiveling frame (the "I-beam") that pivots on a pin attached to the rigid frame of the rest of the locomotive near the trailing front engine driver, so that the whole front engine--drivers, cylinders, lead truck and pilot (the "4-8" of the 4-8-8-4), can swing out to the side on a curve. Now, lengthen the frame to the rear, put a truck where the drivers are (right at the pin) in place of the drivers and add another truck at the back of the swiveling frame, and you'll have what is under one end of this car. Based on what I see in the Kasgro drawing, on this car the I-beam pivots on the same kingpin as the center truck. This means going into a sharp curve, the I-beam pivots like the front of the Big Boy and the leading truck on the I-beam swings out to one side of the car AND pivots in the direction of the curve (like the lead truck on the Big Boy). Since the I-beam pivots in the middle, when this happens the trailing truck on the I-beam swings out to the other side of the car AND ALSO pivots in the direction of the curve. The middle truck just pivots in the direction of the curve, as does the I-beam. The same thing happens in reverse on the other end of the car. This is a double articulation that lets this very long car with lots of wheels to distribute the weight go around a comparatively sharp curve, in the same fashion as the Big Boy. The I-beam and lead truck swing to the side into the curve just like the front of the Big Boy. Let me know if this doesn't help and I'll try again.
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