The issue is the interrelationship between the TTC and Mosaic, once operations start (presumably in demo mode - aka test running).
Reading between the lines, I get the impression that (as with the case with Crosslinx) that Mosaic wasn't expecting to have to deal with TTC inspecting everything to their own standards. and applying their safety requirements.
I assume that Metrolinx's competency (or lack of it) in legal contracting will once again be their undoing with these cases.
My best guess as to what happened:
-Agreements were made in principle about construction between Crosslinx and ML, and ML and the TTC, and these agreements don't completely line up
-Over twenty years, scope, personnel and technology have changed, changing the actual requirements
-Mistakes, sloppy construction and manufacturing errors gradually accumulated, and there was no clear quality control process for all aspects of construction (TTC has different safety standards than ML and Crosslinx)
-Any changes require costs, and all three partners are trying to shove responsibility for changes onto the other two (hence all the lawsuits)
-Money for the project has largely run out, so everyone is focused on minimizing further expenses
-People are avoiding this project because involvement taints you with the stink of failure
-Dougie wants the opening of the line to happen close to an election call, and he's aiming for a spring 2025 election, so it'll open then.