Update:

Pedestrian lane position change.

Modular station ? No, but Train Station Content Creator Pack has 2 island stations. I'd expect there will be more island stations on workshop in the future, since there are more variety of island station networks that can be used. You can also make your own station in-game

https://www.youtube.com/user/exocetau made custom train station build tutorials in-game if you want to check out

Do you know any modular station to subscribe for the station networks?

Not sure what do you mean by that. I'm gonna assume that you're trying to drag station networks in-game and notice that they dont have any platform or station building. These are station networks, that can be used in an asset creation, or they can be used with Railway Replacer -which they are part of- on train stations with island platforms.

how come the Concrete Island Station is invisible? Do I need to subscribe to other assets to make it functional?

Gonna have to be a bit more clear with that explanation. What doesnt show up ? What station are you using to replace tracks of ? Do you have the replacer subbed and enabled ?

why doesn't my platform show up?

Nice!