So...does that mean I'm railroading my players because I use systems that don't specifically give them the option of making up details of the campaign world???
I think we have to be really careful about
making up details of the campaign world.
Here are some examples:
* A player is building his/her PC. S/he writes down some notes about his/her PC's family.
* It's halfway through a session. The GM narrates the PCs arriving at the City of Greyhawk. A PC's established backstory is that she is a native of Greyhawk City. The PC's player says "Let's go and stay with my mum and dad!" The PCs' mum and dad have never been mentioned before by any player or the GM.
* A player announces to the GM "My PC really wants to find a magic sword!" The GM decides to include a magic sword in the next dungeon s/he builds. (A more interesting variant: the player announces "My PC really want so find her grandmother's lost magic sword!", and this is the first time the grandmother or her lost sword have been mentioned by anyone.)
* The action is taking place in a tavern. Not much has been said about the tavern except that it has a common room, where the PC's have been eating and drinking. Something comes up that makes the ceiling of the place relevant. A player says "I pull up a stool and stand on it so I can get a better look at the ceiling."
* A player says "I want to find someone who'll sell me illegal firearms at a fair price". The GM sets a difficulty for a Streetwise check. The player makes the roll and succeeds - the player and GM now start discussing the details of the transaction. (This example is taken straight from Traveller Book 1, 1977.)
* A player wants his PC to catch the blood draining from the neck of a decapitated sorcerer, and asks "Is there a basin or jug in the room?" The GM calls for a Perception check; the check succeeds; so the answer is "Yes, there is." (This example is taken from actual play of Burning Wheel.)
* The GM rolls a 1, and as per the rules of the game a player spends a point to create a Resource that correlates to one of his/her PC's specialties. This rule is taken from Marvel Heroic RP. An example of it in play: the PCs were in the steading of a giant chieftain, and arguing about whether the giants should help them on their quest. The PC warthane looks around to see if their is a giant shaman or advisor who will agree with him - and see that there is! (Mechanically: I roll a 1, the player spends a point and creates a Resource - Giant Shaman who agrees with me - based on his Social specialty.)
* As per the rules of the game, the player spends a point to flat-out stipulate that some bit of fiction s/he cares about obtains. I've never played any RPGs like this, but OGL Conan (based on the 3E D&D mechanics) is an example of one.
There's a pretty wide range of techniques and mechanics (or their absence) in that list. More examples, illustrating further techniques and further mechanics, could be given.
I would say that
if the players in a RPG cannot influence - whether mechanically, or by suggestion, or by stipulation - what the possibility-space is for their action declarations for their PCs, then there is a high likelihood it's a railroad. I see this as very closely connected to
@hawkeyefan's reference, upthread, to the players "blazing their own trail".
I say "high likelihood" deliberately, and not just as a euphemism for
certainly. Eg: in KotB, or ToH, the players cannot influence, either mechanically or by suggestion or by stipulation, the possibility-space of action declarations. That's all set out in the GM's maps and keys. But playing those modules probably won't be a railroad. Rather, and assuming the GM doesn't "cheat" by moving the "tiles" or what is hidden under them, then it is puzzle-solving.
But based on my reading of ENworld and other blogs and my general sense of the zeitgesist, I think that that sort of puzzle-solving RPGing is, today, a minority of play. The default for contemporary play seems to be some variant on the "living, breathing world".