For years I have relied on an adventure-design framework I call clear goals, fluid obstacles.
I tend to agree with dd.stevenson, although I don't think it has to be a strictly social thing - it is part of the metagame understanding about who gets to decide what aspects of the shared fiction.The key difference between a railroad and a sandbox takes place outside of the game, on the social level.
In the "clear goals, fluid obstacles" model who chooses the goals? If it's the GM, the game may or may not be a railroad in the strictest sense, but it's not ultimately a player-driven game.
I agree that if all roads lead to a Rome chosen by the GM, then there is some sort of railroading going on.Railroads can have more than one track and hiding the tracks doesn't take people off of them. If all rails lead to DC, that still precludes the underlying assumptions of a sandbox which allows for meaningful choices and multiple endpoints for a sandbox campaign.
I agree with this too. (And therefore disagree with [MENTION=10479]Mark CMG[/MENTION]'s spectrum claim.)"Railroad" and "sandbox" are not two ends of the same continuum.
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While you can't have a sandbox that is a railroad, not all games that aren't sandboxes are railroads.
A key part of a sandbox is world exploration. You can see this, for instance, in [MENTION=463]S'mon[/MENTION]'s advice about rumours and how they relate to map setups. And also in the multiple posts upthread relating sandbox adventures to location-based modules.
But there are ways to do non-railroads that aren't exploration-focused. Roughly, the players set out clear goals for the play of their PCs; the GM frames the PCs into situations where those goals come into some sort of conflict (with one another, or with NPCs/monsters, or both); and then the game's action resolution system is used to work out what happens. Rinse and repeat (keeping in mind that the players and PCs are likely to evolve over time).
The standard term for this sort of game is "scene-framing". It is non-sandbox but player-driven. It requires different GM prep from the sort of stuff described by [MENTION=463]S'mon[/MENTION].
The best examples I know are many of the old ICE modules and campaign supplements for MERP and Rolemaster. They would typically map a small(-ish) region - including key fortresses, cities etc - and describe and stat out their key inhabitants. Those inhabitants would be given motivations, plans underway, etc that were apt to draw the PCs into adventure (assuming a fairly typical set of motivations for the PCs).What would you expect from a sandbox that is not delivered by an AP, and how would you expect a published sandbox adventure to be laid out? Are there any examples of published sandboxes?
A good example for D&D is the 1989 City of Greyhawk boxed set.