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.
If I could find a functional land vehicle—or amass parts over hundreds of hours of gameplay to get one to run—I would want it to fail almost immediately and explode in a small nuclear fireball.