This doesn't make any sense to me. How is two single-track lines more capacity than one double-track one?

the main focus of medium high speed rail should be to duplicate the route of very high speed rail

Not dublicate. The VHR has less frequent stops, its new line is built much straighter, and away from smaller cities where it doesn't stop. Say, you build a VHR with stops in Philadephia, Harrisburg and Pittsburg, and upgrade the old Pennsy mainline with some cutoffs and tunnels, to run MHS with further stops in Merion, Coatesville, Lancaster, Lewistown, Johnstown and Greensburg.

_{*Lunatic*, n.
One whose delusions are out of fashion.}