Lyxen
Great Old One
Why? A simple dungeon with three rooms, in a chain A---B---C. That's as linear as it gets. But, it's in no way a railroad. A travel scenario where you have to escort the caravan from A to B is 100% linear but is not, in any way, a railroad. Linear adventures abound. Any adventure with time pressure becomes linear. Most mystery scenarios are linear - you have to have a mystery first and the trail of breadcrumbs leading to the reveal can be pretty straight forward. There is a shopping list of great adventures that are linear.
Honestly, all this just comes from your (personal) definition of railroading, right ? If I loop at the OP's definitions (themselves debatable, I agree), a scenario when you can only go A=>B->C is railroading:
(1) GM behavior when the planned scenario requires a particular sequence of events/scenes leading to a particular ending. The GM ensures that it arrives there by a variety of means. This is generally pejorative, but is sometimes defended as valid as long as it is not overused. (2) On the Forge, a purely negative term for GM behavior that breaks the Social Contract via the GM controlling a player-character's decisions or opportunities for decisions.
The scenario requires you to go to C, passing by A then B, so it satisfies the first definition, and it controls the decisions and opportunity for decisions as it provides no alternative.
And it's the same with the "bad faith", it's your perspective that it needs to be done in bad faith.
I'm not saying that your definition is wrong, but, as often, it's the terminology which is causing trouble with the discussion more than the basis of the topics themselves.
I tend to call what you are talking about shepherding. And, sure, we all do it. Sometimes you need that big neon sign that says "ADVENTURE IS THISAWAY!!" And that's fine. Generally not really a problem I think and not really a railroading issue. Mostly, again, because this sort of thing is just basic DMing 101. Without any input from the DM, you wind up with what I've seen called a rowboat sandbox. You're in a rowboat in the middle of the ocean. Sure, you can go any direction you want, but, since there is pretty much the same as here, it's all just random chance.
It doesn't have to be as bad as that, although I agree that it's one of the problems of "extreme sandboxing", unless the DM is really good at improvising. Some people like it when marauding, for example, but my perspective is that you should not expect great intrigue.
It's all a question of degree.
I'm going to disagree here. I've seen some very egregious examples of railroading that were blatantly obvious to everyone at the table. In one case the entire group up and quit on the spot because it got so bad. So, no, it's not a lack of trust. Player's are not stupid, and, if you're like me, you play with a lot of players who also run games of their own. It's not that hard to spot when it's being done in bad faith.
But again, "bad faith of what" ? Why do you suppose the DM is doing it ?
The subtext here of "just trust your DM" is not something I can really get behind when the DM is acting in bad faith. Granted, as you said, if the DM is doing it with good intentions and the group is largely happy with the rails, then, no problems. That's fine. But, again, I don't consider that to be railroading since the DM is not acting in bad faith.
See above about the definition causing problems probably more than the act itself.