I still do not understand how local rail ends up fragmented like this. The Tokyo metropolitan area has like… what? 8 operators? You’d feel like there would be some consolidation and yet.
For example, if you wanted to conform the IRT lines in NYC (the numbered lines + the 42nd Street Shuttle) to the BMT/IND lines, you would need to (at minimum), update every platform (to back the edge away from the train), probably update every place two tracks are parallel and too close together, probably also update some of the routing and tunnels to enable a larger turning radius, and reapprove all of the tracks (including elevated portions) to support heavier trains.
Since all of the lines (barring the 7 and S) overlap at different points, you can't just make these mutually exclusive changes to one line at a time; you would have to shut down the whole system to make the changes, eliminating service to huge areas of the city for months (if not years).
Although during WWII and for some years after all the lines to the south and southwest of Tokyo were under Tokyu Railways (then known as Dai Tokyu, or "Big Tokyu).