I think this is where we agree. To me, it's the conflation of linear -which
@hawkeyefan straight up admits that he doesn't see a difference - with railroad that causes all sorts of problems.
Like you say, AP's are linear. Well, mostly anyway. There are a few more sandboxy AP's out there, but, yeah, if you're doing any sort of "Grand Quest" AP, then, of course it's going to be pretty linear. That's how quests work. But, being on a quest isn't a railroad any more than traveling from A to B is a "railroad" even if you were actually traveling by train.
I mean, pre-flight, there are only so many ways you can travel from London, England to Boston, USA. Pretty much no matter what, you're getting on a boat, and, unless you are REALLY lost, you are not going to travel through a port in Jamaica on the way. That's just the nature of the scenario. In context, linear makes the most sense.
Railroading occurs when the wishes of the players no longer matter. No matter what, X is going to occur and the DM will do anything and everything to ensure that that outcome occurs. Gollum will always be the one to throw the Ring into the Fire and die. You will not use Eagles to travel to Mount Doom. You WILL travel through Moria and Pippen (or Merry, I cannot remember) will ALWAYS trigger the horde of orcs to come and chase you, causing the Balrog encounter. Which must always be resolved by Gandalf dying.
Railroading is closing off perfectly logical and plausible choices for no other reason than the DM wants a specific outcome. That's why it's a form of degenerate play.