^{Galaxy Train from Cross Universe}
The Galaxy engine is a Cross Universe rules engine written in Javascript, mainly for https://battle.crossuniverse.net.
Yes, it is named after Galaxy Train, how could you tell?

Working on the engine is currently easiest by just cloning the repo for Cross Universe Online Battle and using its local version for testing.

Even though the code in this repo is licensed under the MIT License, the copyright for the Cross Universe card game itself belongs to Cross Universe.
That means the MIT license can't cover the game itself, meaning you should also follow Cross Universe's own license, found here:
English: https://crossuniverse.net/tos/
Japanese: https://crossuniverse.jp/約束事/