As the tune plays and auto-scrolls across the tracks, there's a small marker at the top that moves, but the moving vertical line that moves across the tracks is gone. Can I get it back?

Is this happening on all projects? Does it return if you close and reopen the project?
 
Also, is this the behavior you prefer for the marker? The default would be for "Options-->On Stop Rewind to Now Marker" to be selected where the marker doesn't scroll and stopping playback will return you to where you started.

This line ?
Press " X " on your keyboard.

Or this line ?

read previous comment. Rcklln is right on.
 
Read more here:
https://www.cakewalk.com/Documentation?product=SONAR%20X2&language=3&help=Playback.03.html