As the title says. Would be nice if all locomotives linked within the same train shared controls. This way we'll be e.g. able to have another locomotive pushing the back of a long train.

You can actually already do this via locomotive control binds, I know I use it for my chain of 4 electric locomotives, you just have to bind a key to speedup, slowdown, power toggle, and reverse/forward toggle

Well, linking them automatically would be a more elegant solution.
Getting them out of "Shutdown" state could be manual tho.