Quote Originally Posted by Darth Ultron View Post
To say ''a DM has the NPCs and environment react to the PCs' actions'' is exactly the same as saying ''the DM has a railroaded plot''.
The DM reacting to the PCs' actions is exactly the opposite of the DM having a railroaded plot, because in a railroaded plot there's only one "correct" way to proceed, and the PCs have to react to the DM's plans and figure that correct way out. To use your elven magic item example:

Now, the Railroad DM makes the setting and an adventure, including encounters, NPCs, creatures, interesting things and notes. And the DM connects everything together in logical ways. For example the DM makes an elven king and his brother the rebel bandit and some politics and backstory. The DM also makes a note that ''the elven king likes people that respect nature''. And dozens of other things.
This is what a sandbox DM does too; statting up encounters, placing interesting things in the setting, giving NPCs backstories and motivations, and so forth is a constant for both game types. The thing is, though, that none of that is a plot, it's the background. You've described creating Rivendell for the Lord of the Rings campaign, but the plot is what happens when the Fellowship arrives.

In a railroad campaign, one of the things the DM does when creating the above setting and background material is decide how the adventure "should" progress. Let's say that the lost elven magic item is an Excalibur-style "whoever has this item is the true king" item, and the DM decides that PCs should team up with the king's brother to find it, because the king would rather it stay lost, so his preparation assumes that's what will happen: he stats up encounters with the king's guard (because the PCs will be aligned with the bandits), but not encounters with bandits (because they'll be allied with the PCs) and not encounters with elven commoners (because they're irrelevant to the DM's plot).

The PCs show up and whenever they try to do anything but join up with the bandits, they're stymied. The party wants to talk to the king to ask for help finding this item, because he likes nature-lovers and the party includes two rangers and a druid? Oh, the king isn't seeing any non-elven visitors now. The party wants to find an intermediary to speak with the king? Oh, no one will cooperate. The party says screw it, let's just search the forest ourselves? Oh, only the bandit leader knows the forest well enough to have a chance to find it. And so on: the party is steered in the direction the DM wants until they give in or the game folds. That's a railroad.

A sandbox campaign, by contrast, is created without that focus on a single plot. The sandbox DM in this scenario stats up encounters with the king's guard, but also with the bandits, elven commoners, and various non-elves who live in the forest, too, and jots down notes about how they'll react to the PCs if they're on their own, if they're with the king's guard, if they're with the bandits, etc. The PCs show up, and the results of their plans are based not on the DM's desire for a certain plot, but on their actions and character. If the PCs decide they want to talk with the king and ask for his help, the king will probably readily agree if it's a party of elven rangers and druids, probably refuse if it's a party of half-orc barbarians and blighters, and probably require some convincing if it's a bunch of human fighters and clerics. If the PCs decide they want to look around the forest on their own, tromping through the trees won't really get them anywhere but making allies with the native fey, staying away from guard patrols, and so forth will bear fruit. No one particular approach is forced on the PCs, and the campaign doesn't bend around the PCs to make things go the way the DM wants.

And of course both of these campaign styles can be done on-the-fly instead of being prepared as such. If the PCs come up with something the DM hasn't prepared in the Forest Temple where the magic item is stored, so he throws in an unbreakable puzzle door on the spur of the moment and the players have to basically read his mind to come up with the right answer, that's characteristic of a railroad. If the PCs say "So, the king is a high elf and wants to keep his power, and his brother wants to steal the item for himself; are there any wood elves on the other side of the forest who might be able to help us find it without trying to steal it for themselves?" and the DM responds "Uh...yes, yes there are totally wood elves in this forest" and then pulls up some NPCs (pre-statted, from the books, or on the fly) to use for the wood elf tribe, that's characteristic of a sandbox.

The Sandbox can only be blank for a couple minutes, as once the DM starts making things you can't avoid railroad plots, unless the game is pointless and makes no sense. Like when the Pc's just destroy the woods as they are ''so super awesome'', the normal railroad game elf king would be upset and take action. Now the sandboxy elf king will ''react to the PCs' actions'', following the railroading plot the sandbox game is not meant to have...
In a railroad game, the PCs wouldn't be able to destroy the forest because the DM wanted the PCs to join up with the bandits to find the item and overthrow the king. In a sandbox game, destroying the forest changes the current plot from "try to find the ancient elven item" to "screw these elves, burn it all down" and the PCs get to deal with the fallout from pissing off an entire elven kingdom and all the forest's inhabitants, and whether or not they keep looking for the item or high-tail it out of there is up to the party. The game doesn't become a railroad just because NPCs are doing anything at all, otherwise the concept of a "railroad" loses all meaning.