It stands for Natural Environment Professional 2. It is a Mod by Grimes, and a few other people. It is available exclusively on TransportFever.net/ Not on Steam Workshop.

What is this NEP2 that has compatibility issues? The only references I can find tell me it's either an operating standard for ethernet switches used on ships, a certain gene that regulates some organ processes, some sort of whacked-sounding sci-fi philosophy or a blockchain encryption standard -- none of which strike me as being relevant here.

It's really fantastic ! I never thought starry sky would be accomplished !

Amazing..!

Thank You Takashi, now everyone has a TL:DR version of the crucial steps.

It's more destructive than I imagined! Thank you.
For those who are unfamiliar with it like me! 2 steps.
１.ensure 'Debug Menu' is enabled in the Advanced Tab of the Options Menu
２.Add this mod to your save game
３.Once the game begins。 press [ALT GR] + [D]
4.The settings screen will appear, so click "Environment tab" → "Change environment"

If you are unsure, press [ALT GR] + [D]
This is the right ALT key when there are two "ALT keys" on the keyboard.

If you enjoy riding trains in games, you should give it a try! I was amazed.thank you.

Vielen Dank