See, I don't really bother with trying to make the distinction here.
Linear is a plausible set of scenarios where progression is A to B to C. If you are traveling from New York to Boston, it is EXTREMELY unlikely you will pass through Rome. And, frankly, a player who decides that he wants to go to Rome and is prevented from doing so is a different issue from railroading. That's a complete failure of player buy in. Now, if we add a time limit to the scenario, and you must travel from New York to Boston in under 4 hours to stop the assassination of the very important NPC, then, well, you have to fly. You cannot take a train, boat or walk from New York to Boston in under 4 hours.
So, we have a linear scenario where choices are constrained. Is it a railroad? No. It's entirely plausible and no meaningful player choices have been removed. There is only one way to travel that distance in that amount of time. You aren't "forcing" the PC's to fly as opposed to any other choice because, well, there is no other choice.
No, I'm sorry, but the "there's no other choice" bit is a tad circular. Why is there no other choice? The GM says so, because that's how the GM put this challenge together. At least, that's how D&D works, where the GM is the only source of this kind of fiction. Other systems may get there a different way, and avoid this issue.
So, your argument is that some kinds of GM limiting of choice is okay, because, well, it's okay, but other kinds, "meaningful" kinds, aren't. And the way to tell them apart is....
Railroading, OTOH, removes meaningful player choice. As in choices that the players can plausible make in the scenario are being constrained, not because of the facts of the scenario, but because the DM/GM has determined a specific outcome that cannot occur if the choice is allowed.
Mostly agree, but this isn't a special form of play, it's a degenerate form of the exact same thing you posted above -- above, though, the constraints on player choice have been sold to the players and are accepted. The difference, really, between your above limitations and ones that result in railroading are going to be specific to a table because they're exactly the same things, just in different places/strengths.
I'm 100% good with railroading being a degenerate play, but it degenerates from linear play; it's not some different animal.
That's the difference between linear and railroad. In a linear adventure, the PC's could still fail. They could, for whatever reason, miss their flight and fail the mission. The scenario does not have a fixed ending, despite the facts of the scenario limiting choices that the player might make. A railroad will have a fixed result, regardless of any decision the players try to make and any action the players take that conflicts with this DM determined outcome (not scenario determined) will be blocked by the DM.
Does that make better sense?
I'm not sure why failure is somehow special here -- you can have a railroad and still fail. There's nothing about a railroad that prevents failure, in fact, many horror stories featuring railroads result in TPKs and total failure because the GM forces that outcome. This isn't an argument that does anything.
Railroading is just a degenerate form of linear play. It's where you do linear play wrong -- you put the wrong constraints on or you constrain to heavily or you don't get player buy-in to the constraints. One of my my memorable 3.x games as a player was pitched as a railroad -- it was a linear story and we were told that if we buy into the premise and follow it, the promise was that it would be an exciting ride. We did, and it was, and I wouldn't quite call it a railroad (even if the GM pitched it that way). It was no more of a railroad than any WotC AP for 5e, at least.