[Solved] How to export and reuse a MIDI track?

You could save it as a project template instead of a track template. Project templates may contain data in the tracks.
 
Or you could just keep it as a regular project and create new projects from it using the "Save As" function.
 
Or you could create one big clip using the "Bounce to Clip(s)" function and drag the MIDI clip to the desktop, a windows folder or the browser in SONAR. Then drag the clip into new projects.
 
Personally I would use option 1 or 2.

If that is the case, you can copy and paste between projects too.

As one example, I often use two tracks of RealGuitar and there are several important controller settings required.  I created a project with two tracks (one for each instance of RealGuitar) and those tracks have the controller messages and comments (added in the event list) to explain what the various controller values do.  I saved this project as RG Setup.  It is only two bars long. (Note for RG Users: Mode, Chord Position, and Pattern Mode are very important.  Some users don't know about Pattern Mode.)
 
When I start a new project, if I want RealGuitar, I open that setup project, highlight those measures, copy, and then paste into my new project.  Because the project is so small, it only takes seconds.
 
I also have a project I called BASIC that has common instruments like Bass, Guitar, Piano, Drums, etc.  I can open it, do a SAVE AS, and start working.  I have another version called BASIC - VIENNA that has the rhythm section plus a Vienna String section.  I have others as well, but you get the idea.