Let's say as an example that this oncoming Dusk War had been a background atmosphere-setting thing all along in the campaign (maybe it's because all the local soldiery has been called to the front lines that brave adventurers are needed back here closer to town, thus explaining the existence of your party at all), but that your intent going in was that it would in theory never become too relevant to anything because a flash flood was going to end it as soon as it started. But during play the players via their characters have brought the oncoming Dusk War to the fore, made it important to themselves, and based some major decisions around it; all regardless whether or not it was important to you as DM and-or whether you had any interest at all in running a war-front adventure or campaign.
Now you're screwed.
By your definitions you'll be railroading if you have the flood happen anyway even though it's what you'd planned all along.
By at least one other definition upthread (I forget who's) you'll be railroading if you deviate from your original plan, dispense with the flood, and let the war continue.
And by my definition you've in fact been railroaded by your players.
Suppose you invite all your friend around to play a dungeon crawl. But then, just before they turn up, you discover that what you've prepared is actually a hex-based Classic Traveller sub-sector. The I guess you're screwed!
Well, maybe; but the number of times anyone ever accidentally prepared a sub-sector when they were meaning to write up a dungeon level is probably pretty low.
Given my preferences for GMing, world-building etc, why would I inadvertantly do the thing I don't want to do. (Qv my comment upthread that certain sorts of prep aren't very useful for a "say 'yes' or roll the dice", "let it ride", "fail forward" approach to RPGing.)
Furthermore, suppose it happens that (i) I do conceive of this strange flash-flood idea, and then (ii) I abandon it for the reasons I suggest. Why am I screwed because [MENTION=6775031]Saelorn[/MENTION] (I think it was) wouldn't approve? He's not at my table, so why would I care that he characterises what I'm doing as railroading?
So a DM's not allowed to have a game-affecting hidden backstory without being accused of railroading???
The notion of "accusation" seems out of place - it's not a courtroom, and no one who isn't at my table is answerable to me. (And vice versa.)
But yes, I regard that as railroading. That's why I don't do it. I know that others' take a different view. That's why I noted, in the OP, that others would take a different view.
You've agreed upthread that something happening off-screen isn't railroading - I think - such as the Marquis being assassinated in his bed the night before.
My point is that there's no difference if it happens on-screen in a manner that precludes the PCs from doing anything about it
I've consistently agreed with [MENTION=16586]Campbell[/MENTION] about the Marquis example. Whether or not it is railroading depends on whether the death of the Marquis is an
outcome - ie an event of significance relative to the commitments, goals, etc of the players and their PCs - or is part of the framing of a situation. If it's an outcome, then it's railroading whether or not it happen onscreen - and the corollary of this is a motto I've often posted before, nameluy, "No failure offscreen". If it's framing, then it's framing whether the PCs witness it or whether they hear of it (eg in my Dark Sun campaign the PCs heard of the death of the Sorcerer-King of Tyr as the opening moment of the campaign - that's an example of framing).
what happens if (or when) the players/characters happen to steer you toward running something you simply don't want to run?
Then you talk about it and reach a resolution of some sort. That's a social problem. It's not an adjudication problem; and I don't really see the appeal of trying to solve the problem of inconsistent preference by covertly negating certain player choices and amplifying others.