On main page of this addon - click "subscribe".
(and Garry's Mod automatically download and install addon)

In order to run a server, you should use a tool such as GMAD[www.treesoft.dk] in order to extract the gm_metrostroi.bsp file into your SteamCMD (assuming you're using the Source dedicated server) folder. You must make your server mount a public collection containing the Metrostroi addon on the Steam Workshop - which is easy to create yourself - if you wish to use the Metrostroi trains on any map.

Originally posted by ILoveSpaceMoreThanYou:

You must make your server mount a public collection containing the Metrostroi addon on the Steam Workshop if you wish to use the Metrostroi trains on any map.

That is how you make your server use addons. Why are you using the compiled Metrostroi file from GitHub?

The Metrostroi gamemode is only in the file from GitHub. As I said, use the Metrostroi addon in the Workshop if you wish to use Metrostroi on a server, because the GitHub file (gamemode) is incomplete - it's also much easier to use your server with the Workshop addon instead because it will automatically update for you, too. :)