I think what constitutes railroading is simply the PCs actions having no impact on the next step of the adventure. If the party finds of split in the trail in the forest but the DM decides they run into the Old Witch regardless of which direction they choose, that's railroading.
There is the important caveat that the choice in question had to be a significant one. If the party chose a particular fork in the hope of avoiding the witch, then having them encounter her regardless could be railroading. But if the choice of path was utterly arbitrary in the first place, placing the witch on the chosen path doesn't frustrate the party's choice. In other words, if the players weren't trying to have an "impact on the next step of the adventure" at that moment then it isn't railroading to prevent them from having one.
Reynard refers to "PCs". Xetheral refers to "players". I think Xetheral has the right reference, in the context of a discussion about railroading.
We're not talking about the (imagined) causal powers of the (imagined) PCs in the (imagined) gameworld. We're talking about the actual causal powers of actual people - the players - who are sitting at the gaming table.
In my OP I said "By
railroading I mean the GM shaping outcomes to fit a pre-conceived narrative." Is an utterly random or unmotivated choice an
outcome? I tend to agree with Xetheral that it's not. It follows that "We go left" or "We go right", in a context where - from the players' perspective - there is nothing at stake in the choice is not an event of action declaration, any more than "My boots are frilly" is an action declaration. It's just colour and performance.
if a party is exploring a dungeon they haven't scouted and have no knowledge of, and are thus deciding which hallway to take at random, it isn't railroading to use a mirror image layout of dungeon instead of the original, since which way the party went was chance anyway.
I am going to disagree with this, but it may be a disagreement that you (Xetheral) agree with!
In a classic Gyagxian dungeon, although the players may not have any knowledge of the dungeon, they have the
possibility of having knowledge (eg by rumour gathering, scrying/detecting magic, etc). Failure to use that capacity, in the context of Gygaxian dungeoneering, is just a mark of poor play. So at least in a Gygaxian game, changing the dungeon might be shaping an outcome and hence (in my view) railroading.
One of the differences between the Gygaxian dungeon case and the "witch down the forest path" case is that D&D has rarely supported the same sort of information gathering etc in the context of outdoors adventuring (eg due to spell ranges, different conventions around how wildernesses are "stocked" with encounters, etc; I think this is why Luke Crane refers to
"the poorly implemented ideas of the Expert set" in comparison to Moldvay Basic). Thus it's much harder for me to envisage a context in which choosing to have the witch at the end of an arbitrarily chosen forest path would count as railroading.
There is no way to play D&D that doesn't contain rails. The good DM's hide the rails.
I don't think this is true at all. If someone runs White Plume Mountain or Hidden Shrine of Tamoachan as originally conceived and presented, where are the rails? The module parameters are all set in advance; the players make their choices; a mixture of GM judgement call and the outcomes of the dice rolls tell the players whether their PCs live or die.
Or to point to a completely different style of game, I don't think my 4e games have any "rails". Eg in
this session, the PCs reached an understanding with the djinni and Yan-C-Bin, then established powerful evidence that the Dusk War is not about to take place. I didn't know these things were going to take place until the players declared actions for their PCs and we resolved them.
I also don't see why a GM would "hide the rails". Or, more generally, why would a GM hide his/her techniques? In the session I referred to, the PCs track their lost Thundercloud Tower to Yan-C-Bin's palace on the Elemental Chaos. There is no mystery among the players as to why I've made this choice as to where the tower is located: they know that one of the PCs is sworn to the service of Chan, Queen of Good Air Elementals and hence sworn enemy of Yan-C-Bin, who has already tried to tempt that PC to change loyalties; they know that another PC has just recently taken on the mantle of god of imprisonment, and hence has a special interest in ensuring that bound primordials don't escape; etc.
So why would I want the players not to know my reasons for making this choice? I want them to be fully aware of them, so that they see the full scope for engaging the situation via their PCs. The context that informs my choice is what gives the outcomes dramatic significance.