本スレでも話題出てたけど、ついにトンネルが実装されるらしい。

The next planned update for Cities: Skylines will be the most substantial to date, adding new features, gameplay tweaks, improvements and more fun stuff!
Many of you know it as the "tunnels and wall-to-wall buildings update" that will eventually be added to the game for free.

As the update is a lot bigger than what we've previously released it's also going to be in development and QA for a longer time - meaning there's going to be at least a month before it's ready to be launched.

https://forum.paradoxplaza.com/forum/index.php?threads/april-progress-update.851539/

the update is a lot bigger とあるから、結構追加の翻訳も必要になるかも。