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.