Why is it breakthrough for London?

Inside London's new $25B 'Super Tube'
https://edition.cnn.com/travel/article/crossrail-london-2022-launch/index.html
In many cities around the world, it takes much time to move between an edge and opposite side of the city, mainly because traffic jam or lack of through train service. Even when there is a through service, the train would run slowly and stop at many stations in downtown section.
But, inauguration of the cross rail would solve these issues.