As far as i understand it you should be able to crate a rail with a hitbox of that rail and add graphics that makes it look like a smooth wall. Maybe take a look at the cargo ships mod wich creates "water rails" for the ships. Also there is some code in it that prevents normal locomotives...

Should it lock horizontally or vertically, or allow Bezier curves? What do you mean "horizontally or vertically"? Also not that dynamic, wouldn't simple segment fragments make do? They could also use some "look at" time, the shadows don't match with sun position guessed from oth...

TL;DR Title says it all; rail-like draggable and smooth wall building. What ? Walls implemented as rails, which allow for clicking one side of placed wall and dragging it to create angled, rounded, and/or smooth wall sections (but still somewhat 1x1 in size per segment as possible or not) that conn...