I agree with cascaderailroad, while Sketchup is far, far easier to learn and use than 3D graphic design programs like Blender, GMax and 3D Max, there is a severe penalty that you pay for that convenience.
The objects (e.g. locos) created by Sketchup contain far too many "polys" (or polygons - the geometric shapes that are used to construct 3D graphical objects). This reduces the animation display speed ("frame rate" or frames per second) of Trainz/TANE because the GPU needs more time to draw each Sketchup created object on the screen. At 30 frames per second, which many regard as a "good" frame rate, the GPU has to draw every object on the screen (as well as those behind the "camera") 30 times every second. If even one object has an unusually large poly count then it could, depending on the graphic card installed, make it impossible to achieve that moderate frame rate. The result is a very noticeable "stuttering" on the screen.
The same objects created using Blender, GMax or 3D Max have much lower poly counts without reducing the visual quality of the objects themselves. As an added bonus, some of those programs can also create even lower poly versions of the same objects that can be placed further away, using Trainz LOD (level of detail) feature, to even further reduce the work load on the GPU.
For these reasons, Sketchup created objects are not popular in the Trainz community.