Grand Paris Express will be 200 km of new track to provide massive new connectivity to the Paris suburbs. Some extends existing lines, most is completely new lines adding connectivity between RER lines much further from central Paris to enable many trips to avoid the centre of the city. It's a combination of new tunnels and elevated guideway to be fully grade separated.
Closer to home I'd point to Montreal REM which will be 67 km of track in new elevated guideway when it's all done using automated trains. Phase 1 opened last summer. They did re-use an existing tunnel through downtown, but it needed extensive repairs and reconditioning as it hadn't been is use for nearly a century.
If you're interested in what systems around the world look like and what's been possible to build I'd highly recommend RMTransit. Canadian like me, so many videos are focused here, but he has tons of explainers on metro and train systems all over the world.