I feel like often people are misusing the term railroad to describe anything that isn't a complete and total "let's make it up as we go along" style of play.
If the GM is the one who is deciding what is happening, and what's at stake, then I regard it as a railroad. The fact that the players might be making some decisions - eg to go west rather than east - won't change that, if it is still the GM who is deciding what is happening and what is at stake.
If the players can make meaningful choices about how to take risks and establish stakes - as in a certain sort of classic dungeon-crawling - then it's not railroading. But it's very hard to achieve this sort of play in a larger, non-dungeon framework.
I read
Masks back in the day. It's basically a breadcumb trail of clues that take the party from one location to the next. As
@Quickleaf says, linear, rather than a railroad. There is nothing to prevent the PCs giving up and going home, apart from the world will end some time later (and they don't know this).
The players can't have their PCs
give up and go home if the GM won't present relevant opportunities and resolve such action declarations.
There are a lot of times that its possible to have linear adventures that you don't have to "railroad" the players to follow... if the players' most logical next set of actions take them in the direction the module expected them to go. A DM doesn't have to force their players down a specific path if the players just decide to follow that path on their own.
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if they didn't want to follow the breadcrumbs given in an established module, they should have told the DM beforehand that they wanted to just "walk around" and then get random encounters plopped in front of them.
"Follow the GM's breadcrumbs" and "walk around and get random encounters" are not the only two possibilities. The GM can follow the players' "breadcrumbs".
I think this is a really good point. The most common thing I've seen when people talk about railroading in an adventure, they're really saying they didn't want to engage with the premise of the adventure.
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But you're talking about Illusionism here. I guess I'll say that if all the choices I make made for a fun evening, how much does it matter that I was going to fight ogres no matter which way I turned? If the ogres were interesting, I'll take it. But I'll give a huge caveat: an adventure like this is akin to going to a magic show. You know magic is just sleight of hand, but you'll enjoy the show a lot more if you don't deconstruct each trick.
It's not
illusionism if there is no illusion.
Who is in charge of establishing what scenes the PCs find themselves in - the players, or the GM? If everyone at the table knows the answer to this, then there's no illusion.
And what is the basis on which those scenes are framed? And what is at stake in those scenes? If everyone at the table knows the answers to these, there's no illusion. And if it is
the players who exert significant or principal influence over the logic or the stakes of the scene-framing, or if those things are part of the agreed game (say in a
knight errantry or
martial arts schools game), then there is no railroading.
Conversely, when the GM keeps the basis of scene-framing and the stakes secret, and under their unilateral control, then there is railroading and possibly illusionism also.
Relating this to the "quantum ogre" - we can't say anything meaningful about the "quantum ogre", as far as illusionism and railroading are concerned, until we know how those decisions about framing and stakes are being made.
Just as one illustration of the point: If the GM is following the players' "breadcrumbs" - so placing the ogre is not the GM's imposition of their vision, but the GM responding to the players' vision - then there is no illusion (everyone knows why the ogre is being made a part of play) and no railroading (everyone knows why the ogre matters and what is at stake).