I think the term ‘railroading’ is most useful when it is confined to describing a very particular mismatch of expectations and desires that long haunted tabletop RPGs. This is the mismatch between having, on the one hand, a Game Master who expects the players to play through his or her carefully prepared story; and having, on the other hand, players who expect or at least desire to make decisions for their characters that significantly shape the story. Obviously this combination is impossible. But there have been countless roleplaying games which promised the players unlimited freedom to shape the story, and at the same time instructed the GM to pre-plan a story (or buy a pre-planned one in the form of a supplement). In those circumstances, it can seem as if the job of the GM is to carefully guide those pesky players – who are never just going along! – back to the story. This is railroading. And the best GMs are those that make the pre-planned story appear to be the result of the players’ choices, even though it is not – this is illusionism.
Of course, players frustrated with railroading tend to do bizarre stuff to break the story, and then the GM has to work ever harder to get them to where they need to go, and everybody ends up having no fun at all. There are some amazing adventure supplements from the early days of D&D where the GM is given a bunch of increasingly forceful ways for getting the players to, say, the necromancer’s island in case they don’t simply accept the quest in the tavern. Which raises the question: why wouldn’t they accept? And the only answer can be the mutual frustration resulting from the basic mismatch of expectations and desires.
Outside such a situation, the term ‘railroading’ quickly loses its usefulness. I think it’s helpful to approach this in terms of what kind of agency players expect, and what kind of agency they get. In a standard D&D campaign, players neither get nor expect agency over the larger story – after all, the Dungeon Master paid $50 for that new adventure taking you from level 6 to level 10, so you’d better follow that adventure! That’s the ride you’re here for. Instead, players get and expect agency in tactical combat (and sometimes in skill-based ‘roleplay’ encounters). Perfect. There could only be railroading here if the DM actually pre-cooked the fights in such a way that they could only be won one way, because that’s the only dimension where expected player agency could be taken away. Whether or not the story could branch in different directions is irrelevant.
As an IF example, compare a typical Choice of Games offering (with its widely branching paths and many endings) to Turandot (with its single path and ending). Is the second railroaded? Not at all. The typical Choice of Games piece offers the player the opportunity to create a character and pursue in-character character-dependent goals, with some chance of failure but a fairly high chance of success. This requires a branching structure with many ends. Turandot offers the player the opportunity to inhabit a very specific pre-created character and perform him as he undertakes what will turn out to be a quest of self-discovery. The game never takes away your opportunity to perform him. Arguably, a branching story-line with chances of success and failure would lessen player agency in this particular game. (It is of the essence that you can follow your instincts as a performer and engage in antics so ridiculous that you fall into the crocodile pit and come out unscathed, ready to continue the romance.)
In other words: railroading is a term used to describe the trivialisation of player choices in a situation where the expected/desired form of agency is control over the story, hence, where this trivialisation takes the form of always forcing the narrative back on a pre-conceived trail. The problem with the term is that it emphasises the form taken by the trivialisation, rather than the reason that this is a problem. This may fool us into thinking that a linear narrative is something bad; or that taking player choice seriously requires branching storylines. But that’s not true! What you need to think through, and communicate to your players, is what agency you want the players to have, and make sure you give them that kind of agency.
(This actually seems like a good topic for a longer essay… hm…)