The maglev tunnels were part of the transcontinental system between states, known as the Maglev Network. Just like the Net, after the Unification Wars, they crumbled as they were no longer being used.
Night Corp has plans to revitalize the old maglev network.
People who have never been to Night City might be forgiven for getting NCART and the maglev tunnels confused. NCART is the city's public transport system, which includes the metro, while the tunnels are part of a still-under-construction high-speed rail network. They also represent two distinct social groups: the crowded, stifling NCART trains lug the lower and middle classes around, while a ride on the maglev (once the lines are opened, of course) will be for only the highest-ranking corpos.
The existing tunnel, which will one day connect Corpo Plaza to the East Coast, is currently being renovated and expanded. Its opening will hail the grand return of the maglev network to Night City following years of neglect, when old stations were either abandoned or taken over by city gangs.