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 feel like it wouldn’t be that difficult to largely limit vehicles to a segment of the map. Even if they didn’t want to set a hard boundary like a fenced border with Fallout 4 levels of clutter on the road they could be made impractical for certain areas. I don’t think that would necessarily make the map feel smaller or mess with combat.
I think the big concern though would be people pushing them into locations they were intended like Skyrim horses.
I know what you’re talking about, however, I think that any kind of limit that they could practically set in order for it not to screw with gameplay too much would kind of suck all the fun out of it, defeating a purpose. Ultimately, there would need to be some sort of balance, and I think that balance might just be too hard to find.
It might be interesting to experiment with it though.
That’s probably how they just settled on the teleportation device.