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.
I want a small nuclear submarine. That would open up so many exploration options. You could zoom around short distances manually or use it to fast travel long distances from port to port (say DC to Boston). Mutated sea creatures to run from or fight. Flooded sunken coastal cityscapes. Ghoul Chinese sub crews still fighting the war. Secret underwater research stations. So many options.