Except in this case the players (actors) largely improvise the script, to a greater or lesser degree also improvise the plot, and choose which bits of the scenery they interact with and-or play in front of; which makes it not a railroad.
But the only "plot" that may be allowed is one that uses the pieces that exist in the world. The only "script" that may be allowed is one that uses those pieces. That's the problem here.
If I may, allow me to use a hyperbolic hypothetical to demonstrate how a DM/director/"referee" could claim that the players/actors have the freedom to improvise without really having that much actual freedom. Consider a situation where you, as a player, are only allowed to quote from Bible passages that have been pre-approved for quotation. You
technically still have freedom to "largely improvise" the script/plot...but you're not really in the driver's seat, are you? It would take extreme, near-genius level creativity to dynamically turn such Bible passage quotation into something personal and individual, even though you
technically have the freedom to quote any part you want.
Now, this is pretty obviously extreme. I sincerely doubt any DM has ever done something actually like that, where players are literally only allowed to
quote from things someone else wrote. I certainly don't believe anyone here, even folks whom I think DO go overboard, has ever done this. So, to be absolutely, unequivocally clear,
this is not an accusation of any kind, I don't believe real people do this, and I certainly don't think anyone here does it. The point, as stated, is to show that at some point, the DM taking too much control over setting does in fact take away the players' ability to "largely improvize the script...or...the plot", because "choos[ing] which bits of the scenery they interact with and-or play in front of" isn't enough, by itself, to avert a railroad. To use the Disney theme park variation of the railroad concept, just because you can take pictures with Mickey and decide which rides you ride on doesn't mean you aren't on rails or walking pre-determined paths 99% of the time.
Conversely, I think we're all agreed that player improvisation necessarily has to be present when DM's setting material is pretty soft-touch or sparse. Not because anything about that side is better (I love me some great worldbuilding!), but simply because...the players
have to be improvising just to participate at that point. Meaning, if we grant that the above (intentionally extreme example simply to show that
at some point player improvisation is clearly completely taken away by excessive DM setting control), then there must be some point or range where DM setting control begins to outweigh player improvisation.
Only by your definition of what constitutes a railroaded game, which from what I can glean over time seems to include pretty much anything where the players aren't also setting authors.
That certainly does not seem to be the case to me, even without
@pemerton having explicitly rejected this in one of his own replies to you.