The Spadina Extension was IIRC about 1.5-2 years late for example - and that was not particularly notable for a project of that scale. The media still played it up.
It's not "accountability" if basically every construction project runs into delays. It's like doing a home renovation and calling the homeowners "incompetent" and "needing to be held accountable" when a surprise batch of mold or structural repair is found when the drywall is removed. No matter how much you plan, you just don't know what's going to happen sometimes until you get underway.
Just to clarify on this point, Two TTC staff lost their jobs, fired by Andy Byford over the way this project (TYSSE) was being managed.
So there was a measure of accountability and someone generally well thought of in the transit industry insisted on as much.
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More broadly, project schedules are supposed to include both financial and temporal contingency. A project of scale, for argument's sake a 4-year expected build, will clearly have weather impediments from time to time, which may well exceed a typical 'average' year. The odd trade strike needs to be accounted for too.
But that's supposed to be baked into the schedule, that 4-year project, run optimally, is actually a 3 year - 6 month project with 6 months of contingency baked in.
So when we're talking about slippage here, we're talking about slippage beyond a generous contingency.
As someone with a bit of insight into some of the disputes here (much is public in the past litigation), yes there were arguably unforeseeable circumstances, and Covid beyond that.........but there were plenty of mistakes, and things done out of order that created significant problems.
I would add, Finch while better run has had more than its share of problems as well. Many of these stem from very poor planning, and suspect project management.