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 motorcycle mod and the mobile base truck mod on the New Vegas nexus were very promising, but they had some issues and were susceptible to crashes due to being script-heavy. You could even mount guns on the motorcycle! It really fit the wasteland aesthetic. However the environments were mostly made to be crossed on foot, and not by a vehicle which can only be perfectly flat and only move in straight lines. Definitely recommend.