I dont think you should need to decimate the model as it is ... just select the tracks as in my picture and press "P", then choose "selection" from the roll-out menu that appears. This is actually creating another mesh, where the selected vertices will be moved to and the base-mesh will be not having them anymore. (do it to all those track-stripes -- front left inner, front left outer ... etc.) - You see, the limit is actually not on the model itself, but just on single meshes, of witch you can have hundreds, so if you split a model into several meshes of few vertices this way, the importer will handle it. Off course, sometimes decimating or manually joining some vertices into one is a good choice, but as i say, i dont thing you need to do that if ya do the "splitting" into several meshes for this particular model.
.. yes i forgot ... you probably see just one thing at import too !
The thing is, importer needs some mesh-dependency. Something that says it witch mesh is coupled to some other. Overall its called a "mesh-tree". You can create it by "parenting" meshes together. In order to spawn the entire model (and not just pieces of it later in OT after importing), you need to parent all the parts to some single mesh (lets call it a base mesh), witch it would best if its called the way you want the model be named. This way you find the model fast in the importer and spawn windows in OT and it will spawn all of the meshes at once.
Also, in the little sub-window top-right in blender (where the camerra and light bulb is), you can see the mesh-tree and see, what is parented to what. Its good to have that window stretched up to better see whats in there ...
P.S.: im not sure if my blender imported the model badly, but it seems to lack the for the track needed wheels.