I don't know what work is happening this weekend, so I can't respond with facts - nor will I defend ML's performance in execution - but this simply isn't so in many cases, Running live trains through a work zone complicates activities immensely, even if the tracks are useable. Especially for ML where so many contractors and sub-contractors may be involved. Productivity is very much impacted.
Maintenance shutdowns pretty much ground some other countries' rail service on Sunday mornings. Of course, they may have more routes where diversions or alternate routings are possible.
Considering the urgency of getting new infrastructure in place on the GO network, I would say that the shutdowns are worthwhile pain. Over the longer term, perhaps as competencies grow and rules are changed, this will be streamlined.
- Paul