Of course railroading is not a grammatical antonym of sandbox. Neither do a lot of things bear such relationships to "democracy", or "free enterprise", or "marital fidelity" or what have you, that are nonetheless opposed in pretty basic ways to the ethos of the undertaking.
Well, this raises an interesting point I decided to gloss over in my original message for fear it would interfere with clarity.
Could you have a sandbox campaign (in which players are completely free to choose whatever scenario they want) in which the actual scenarios are heavily railroaded?
I think that's an unlikely possibility. Since, as you note, non-linear scenario design and sandbox campaigns share the commonality of enabling/promoting player choice. And if that's what you like, you're unlikely to promote it in scenario selection and then work against it in scenario design. And, as I noted in my original message, I think we tend to see railroading "creep in" from scenario design down to individual decision points rather than vice versa. It's easy to imagine someone controlling macro-level events while leaving micro-level events undetermined; but it's more difficult to imagine a scneario in which micro-level events are being railroaded while macro-level events aren't.
OTOH, it seems at least plausible that such a campaign
could exist. And I would actually go so far as to say that if you translated most video game sandboxes directly into tabletop games, the result would be very light sandboxes with incredibly railroaded scenarios.
It also brings up an interesting point that I don't specifically remember seeing before. Usually I've seen "railroading" include a break with versimilitude.
I'm going to dispute the premise. For example, one of the most-cited examples of a railroaded adventure is the "PCs must be captured" sequence form the A series of modules. There's nothing inherently unbelievable about the PCs failing to detect a
wall of force trap and then failing their saving throws against the subsequent poison gas attack. It doesn't violate versimilitude, but it doesn't change the railroad-y nature of the encounter.
Similarly illusionism, the practice of "invisible" railroading, would seem to depend on verisimilitude to remain undetected.
Re: The general form of "you must do X or your character will be automatically killed". It's a false choice. We could probably come up with some oddball corner-cases where it's a meaningful choice for a character who has some motivation for seeing the world end, but in practical term it's no different than "you must do X or we'll stop playing the game". It's an in-world version of "this is the scenario I prepped, so we either play this or we play nothing".
Which is fine. Lots of people play the game that way. Heck, I ran a "this is what we're playing" session just last night.
But it's still railroading.