Monorail is a fork of Google Code, which was Google's public source control offering for a while.

Repos from Google Code were mostly transitioned onto more popular platforms (like GitHub). Chromium wanted to keep using the issue tracker, so they took it over to create Monorail:

https://chromium.googlesource.com/infra/infra/+/refs/heads/m...

Chrome is the biggest one, but many of Google's public-facing issue trackers are on Monorail (which is hosted at bugs.chromium.org).