It's free. Follow the link in my post, then the link in the Wiki and you'll get there. Since posting that I have imported some Sketchup models into GMax and then into Trainz for my own use without any major problems.
It works in the sense that I have imported a few models from the 3D warehouse into GMax, redone the textures (their transfer is rather hit and miss) and then exported to Trainz. I don't use Sketchup as such, as it is so poly hungry compared to GMax.