I started a Garratt Engine years ago. This was my main drawing I worked from. The drawing shows it in narrow gage, but my research showed it was designed with the intent to easily convert to standard, so I modeled it in standard gage.
The original thread is here viewtopic.php?f=22&t=1133
In the zip is the original full size model I did in sketchup, and I'm including the two most recent blender files I had. I don't yet have blender on this machine so I can't look at them to see what they are specifically.
Note that I had some difficulty getting the sketchup file, modeled in real world size, imported into the blender file in the scale necessary for the game, if you have to revisit that conversion.
I was told the game only supports two pivot points, so they are the joints connecting the boiler-chassis to the two motive chassis. I set-up the main bones for articulation and I tied the motion of the leading and trailing trucks of each motive chassis to the movement of the motive chassis relative to the boiler/cab-chassis. I'm not sure if the game would support that sort of linkage or not.
It still needs bones for other features and functions.
Use these files however you wish, I likely won't have time to ever finish the project, so better if it passes on to someone who will.
Thanks