I use the word functional instead of usable or controllable because that’s kind of under the umbrella of how they could be implemented.
Both Fallout 3 and Fallout 4 had rail systems you could see moving (the presidential metro and Nuka-Express) opposed to Fallout New Vegas (Camp McCarran’s monorail) for example.
I feel like the two major concerns would be how their addition would effect map design and how the engine is going to handle them.
The core of the game is like any other Fallout, but you can see other vault dwellers and conquer the wasteland with them if you want.
There is a lot of grinding if you want to get stuff for your base or certain weapons, but you can play the game entirely solo, except for public events where other players will be working towards the same goal as you to complete a loosely timed mission. The multiplayer aspect can be as relevant as you want. Most of the other players are nice and greifing is mostly limited to bases that are designed to kill you, but you can never interact with other players and never go to their bases if you choose. The only consequence to death is dropping your scrap junk that other players may loot. Public voice chat is abnormal, most players communicate entirely through emotes and actions like shooting at things to call attention to it(like something you intentionally dropped or to alert another player that you are waiting for a trader that they are occupying.)
There definitely is an incentive to pay for the monthly subscription, called Fallout 1st, that gives you a lot more inventory space and exclusive items that are inconsequential to playing the game. You don’t need the subscription to enjoy the game, but will have to manage your limited inventory storage and can’t loot goblin to your hearts content because of that.