MaxisGuillaume has released a new SimCity mod that allows the Cities of Tomorrow MagLev station to be placed underground.
This mod add a new set of MagLev tracks and stations to the game, so they mimic a Subway system rather than an elevated rail one.
The stations do not need the Academy or ControlNet in order to get unlocked and running. They do not futurize the areas around them.
You will need to have Cities of Tomorrow in order to see the stations and tracks in the MagLev palette.
Much like for the MagLev system, you’ll need to place the stations on roads first and then connect the tracks between them.