This is a design and built based on info provided by the city, Metrolinx and TTC, but you can only factor in so much for unknown before pricing yourself out of the picture on that info.I suspect that the bidders who low balled ie crosslinx, bid on incomplete drawings and did not factor much for extras. They surprise everyone with an amazing number, but as shown, once changes are needed the dollars just keep on piling up.
Source: I work in the buildng construction consultant industry
We have just seen a 1950 error surface unknown to TTC and everyone that will have to be added to the bill. It possible TTC knew about it and hope the low bidder would have to pickup the tab for the cost to fix it. My experience says Metrolinx will be on the hock for that cost since no bidder would assume there would be a structural problem during the bidding stage.
COVID-19 delays is not the responsibility of anyone, but a fact of life where cost will increase over time as well delay the project. If ML wants x opening day that requires more manpower, then they will have to pickup the extra cost to meet that date.
Unless plans are updated as work take place over the years on things, you will run into very few surprise that will cost more to do the work. One can look at St Clair W stations where drawings were not updated over the years only to have the Ministry of Labour shut down the site until all the electrical system drawings were updated after a number of workers were almost kill doing the work. Extra cost had to be pickup by TTC.
I have bid on x project based on drawing supply to all bidders only to find out once you open something up a great surprise that caused delays and extra cost is found.