I think there's a lot to say about "railroading" from the GM's standpoint, from the designer's standpoint, and from a sheer definitional standpoint, before we can consider the matter to be understood, and our understanding to be robust. I don't want to hijack the existing threads which focus more on player experience, so I'm starting this one for GMs to post and discuss definitions, techniques, applications, theories and examples of what some call "railroading" - as a literary and game-epistemological device used by both designers and GMs.
Important: Please note that when I say "GM" here I am speaking strictly of single-GM-run worlds and game systems. I am NOT speaking of "the loose set of functions which can be divided amongst the group". I do not need education on the fact that those functions can be distributed in some games. When I say "GM" in this thread I am speaking about one person: the only person at the table who (on my view) is not playing a game, but providing a service. Don't argue with that sentence; you know who I mean.