OK, so like I thought, not exactly an apples-to-apples comparison to measure the DCTA line against the Tex Rail line. The Carrollton to Lewisville portion of DCTA is analagous to the Tower 18 to Grapevine portion of Tex Rail, though. Since that segment has been DART ROW for many years, I doubt that was a major source of the difficulty this project has faced.
Not in the construction stage because they will just basically replace the existing tracks with new, adding stations platforms, and more double tracks for passing sidings. But once operations start, sharing this section of mostly single track with freight trains will create scheduling headaches. The sections south of Tower 60 will be easily to schedule as these trains will not be sharing tracks with freight trains, only sharing the tracks with Amtrak and TRE trains. But these dedicated tracks will be more expensive to build.