Railroading can take myriad of forms, but it tends to boil down into a few major methods.
ENFORCING FAILURE: The PCs cast a teleport spell to bypass the frost giants guarding the Necromancer’s inner sanctum (that’s the player choice), but you want them to meet the Frost Giant King (that’s the preconceived outcome), so you decide to add a teleport interdiction field to the inner sanctum (that’s enforcing failure). The wizard casts a finger of death on the Frost Giant King (that’s the player choice), but you need the king to escape so that he can wage war against the human kingdoms in Act III (that’s the preconceived outcome), so you fudge his saving throw to keep him alive (that’s the enforced failure).
In its extreme form, enforcing failure becomes pixelbitching: Absolutely nothing works except for the precise preconceived path that the GM envisioned for the adventure, so the PCs are left to fail and fail and fail again until they finally guess what the GM wants them to do. (The term comes from poorly designed computer adventure games where players had to guess which specific portion of the screen the designer wanted them to click on.)
Enforcing failure can also crop up in adventure design, but it’s a gray area: Bank vaults are supposed to be difficult to steal from. It’s not railroading if you design a bank vault with a big heavy door and a really difficult lock. It’s not even enough to look at the intention behind those design decisions (because, of course, your intention with the big heavy door and the really difficult lock is, in fact, to prevent the PCs from accessing the vault in certain ways). You have to look at the totality of the design here: If your scenario is designed in such a way that the predesigned world is constantly funneling the PCs into a single course of action, you’ve probably got a problem.
Generally speaking, designing your scenario from the perspective of the game world (instead of from the perspective of how to control your PCs and guarantee specific outcomes) generally solves the problem.
Another gray area arises when failure is enforced on a macro-scale: If the PCs successfully take an action that should prevent a specific event from happening, but then the event happens anyway, that’s a form of enforcing failure. For example, let’s say that the PCs are serving as bodyguards for Lord Harrow. Lady Karna sends a squad of orcs to kidnap Lord Harrow and the PCs defeat them. But you really need Lord Harrow kidnapped, so you decide to add another squad of orcs who pop out and grab him. That’s railroading.
But if you’re dealing with smart, active opposition that are trying to make a specific event happen, the fact that they keep trying to make it happen even after you’ve stopped them once isn’t railroading. For example, if Lady Karna responds to the PCs defeating her orcs by sending a different kidnapping team the next day that’s not railroading.
FALSE CHOICE: “We take the corridor on the right.” “You find the Altar of Despair.” [REWIND] “We take the corridor on the left.” “You find the Altar of Despair.”
This is similar to the magician’s choice: The PCs appear to have a free choice between multiple options, but no matter which choice they make the GM forces the outcome he wanted the PCs to choose. (For example, the magician asks the spectator to choose between two cards. If the spectator chooses card A he says, “Flip that card over!” If the spectator chooses card B he says, “Okay, we put that card back in the deck. Flip the other card over.”)
Location-based false choices are quite common: No matter where you go, you end up in the same place.
Event-based false choices are also quite prevalent, although they can run into the gray area of smart opposition. (Lady Karna’s kidnapping teams finding Lord Harrow a second time is railroading if you’ve simply negated whatever security precautions the PCs have made to prevent that from happening. But Lady Karna really wants to find Lord Harrow, so it’s reasonable for her to have invested the cash in a scrying spell if the PCs have exhausted all of her mundane efforts to locate him.)
Another variant of the false choice is the timeline that doesn’t matter. In published scenarios, this seems to often show up in the form of, “No matter how long it takes the PCs to reach the Lost City of Bakkanar, the bad guys have gotten there just before them.” Here, though, you can see the gray area between enforcing failures and false choices: Is this a false choice because it doesn’t matter whether they travel by ship or horse or teleport spell? Or are you enforcing failure by claiming that their effort to use a teleport spell to outpace the bad guys automatically doesn’t work? It doesn’t really matter, of course. It’s railroading either way.
PROMPTING: Prompting is not, strictly speaking, a method of railroading, but it’s a related technique. It basically boils down to the GM making suggestions about what the PCs should be doing, either directly or indirectly. A common variant is the GM NPC who tags along with the group and either tells the PCs what to do or hints at it. GMs can also prompt in the metagame by simply telling the players what their characters are supposed to be doing.
A more complicated form of prompting is simply having negative consequences from failing to take a particular action: Something horrible happens, informing the PCs that they should have taken action to prevent it. If they still have the opportunity to undo it (or stop it from becoming worse), then that’s serving the same function as a nudge. For example, they hear a rumor that there’s a Necromancer in the forest that’s using a bane bone to create skeletons. They ignore it and a week later a group of skeletons attacks the village. They continue to ignore it and the skeletal attacks become larger and more frequent until, finally, they return from the dungeon one day to discover that the Necromancer has taken over the whole village and reared a tower of bone in the town square.
Some prompting has a generally negative effect on the game (because it tends to influence or corrupt the players’ decision-making process). Other prompting has a generally positive effect on the game. But prompting, by itself, isn’t railroading.
Where it intersects railroading, however, is when the GM is using the prompt to tell the players what the GM’s preconceived course of action is. The GM is basically saying, “You should be doing this. If you try anything else, it’s going to fail because this is the thing that you should be doing.”
MIND CONTROL: Mind control is a special case. When a PC has their mind controlled, the GM isn’t negating the outcome of a choice. Instead, they are removing the player’s agency entirely. It’s a complicated issue that deserves a dedicated discussion of its own.
As far as railroading is concerned, my opinion is that mind control is not necessarily railroading in much the same way that failing to hit an ogre with your sword isn’t. Where it becomes railroading is if the GM uses mind control in order to create a preconceived outcome. (Although here, again, there’s a legitimate gray area: It’s not unreasonable for Lady Karna to use a dominate person spell on a PC in order to have them lead her kidnapping squad to Lord Harrow’s hiding place.)
If I was looking for a litmus test here, it would be the GM’s willingness to fudge the PC’s saving throw in order to make sure the mind control happened. That’s a dead giveaway that they’re committed to their preconception.
GUARANTEEING SUCCESS: Is it railroading to guarantee that the PCs succeed at what they want to succeed at?
I don’t necessarily have a satisfactory answer to that question. For the sake of argument, let’s lay aside the issue of game mechanics that guarantee success for the moment (whether that’s universal mechanics like GUMSHOE’s automatic clue-finding or specific instances like a character who’s skill is so high that they’ll automatically succeed on a particular task) and simplify the discussion by simply talking about a GM fudging a die roll to make a PC succeed where they should have failed.
Is that railroading?
My gut says that it isn’t. There is a fundamental difference in kind between negating a player’s choice and enabling that choice.
With that being said, however, the guaranteeing of success does share a number of negative traits in common with railroading: It flattens the gaming experience by making it more predictable. It removes complexity and interest from the scenario. And it can significantly warp the decision-making process of the players.
This becomes particularly true if the GM shows favoritism towards enabling only certain types of decisions. (Which should become really obvious if you consider that one type of decision that can be selectively enabled is “decisions which lead to my preconceived outcome”.)
INVISIBLE RAILROADS vs. VISIBLE RAILROADS
A categorical distinction is often drawn between visible railroads (where the players can see the tracks) and invisible railroads (where the mechanisms are hidden from them). Invisible railroads are sometimes referred to as illusionism (referring to the fact that the players only have the illusion of free choice), and the simplest example would be fudging a die roll behind the GM’s screen. In any case, the difference between a visible railroad and invisible railroad is more of a spectrum.
Invisible railroads are often invoked as another excuse for railroading:
It doesn’t matter if the players don’t know they’re being railroaded!
In practice, however, I would note a couple of things.
First, GMs tend to overestimate the degree to which their players don’t notice their railroads. Lots of players are polite enough not to pull back the curtain on the Wizard of Oz, but that doesn’t mean that they don’t see his feet poking out from behind the curtain.
Second, the majority of negative effects created by a railroad exist whether the players are aware of the railroad or not.