Railroading
"It's the only way."
Railroading occurs when the GM has a particular story or sequence of events planned out and will not allow the players to deviate from it. A less-insulting synonym is the "linear" adventure; antonym is the "sandbox". Or, back to insults: Calvinball.
Where the campaign has a story at all, some small amount of railroading must occur - especially smaller one-shot encounters - and is considered reasonable and necessary, considering no one amongst humans are psychics (at least not yet) and cannot guess what to do all the time. Excessive railroading, however, destroys the illusion of freedom that RPGs provide and alienates the players.
For this reason it is important that a GM be able to improvise when the players go off the planned trail without simply forcing them back onto it again. Where the story has to get from A to B, a nimble GM must accommodate the players' decisions, allowing for occasional zigs and zags, or arranging such that the apparent detours still end up where the story must lead.
Orrrr the GM can ... plan ahead. A common design pattern in longer-form published adventures is to start the action with a linear plot, open to a sandbox in the middle, and bring the story back to linearity for the endgame. This is particularly notable for Monte Cook's remake of Temple of Elemental Evil which, at its end, simply buries the bulk of the classic Temple under rubble excepting where Monte wants you to go. The middle part of that remake occurs somewhere else, though; so remains a sandbox.
If you feel you're being excessively railroaded, it is common to make a "Choo choo!" noise, if you are a jerk, or other train related sounds. Alternatively, you can just pull a Henderson.
Spotting the Railroader
If a GM is really good, you'll never know that you've been railroaded, as you'll think that your roleplaying group has reached or overcome whatever situations occur naturally.
Most railroading GM's are not like that, though, and are typified by sets of behaviour that makes it quite easy to spot.
They talk... like a LOT... though they might not be fantastic orators, they just like giving you information, both in and out-of-character. This is basically just to make certain that you don't leave the rails they have provided for you.
So, when the group comes to a halt to face a conundrum, most other GMs might be happy enough just to restrict their input to whatever your characters already know, or re-iterate what the players might have forgotten from previous adventures. That way, the solution the players come to will be their own, and they will face the natural consequences from the lack of information, or from whatever hasty course of action they decide to take.
By contrast, the railroading GM will be there making a decision with you, talking you through it so that you completely understand the options available to you, and the consequences of choosing the right answer. There are no hasty decisions with the railroading GM.
Railroading by a GM can become almost painful as the group gets bogged down by indecision, not between party members (which is what usually happens in active roleplaying groups) but because the GM keeps countering whatever decision your group does make by providing a little tit-bit of information piece by piece until you start following the line of story that he has prepared for you.
Below are some examples.
- "Your character remembers something..." i.e.: I'm telling you something you would never have guessed by yourself, but I couldn't be bothered telling you before now. You should now change your previous decision and do what I tell you.
- "You suddenly notice..." i.e.: despite the fact that you've already searched the entire area and found all the information I gave you first time around, you are still not on the path I want you travel, so here's another clue to reinforce my desire that you go down this path.
- "[NPC] pipes in with helpful information" - i.e.: despite that fact the NPC might have been a nameless minion, I have waited until just now to inform you that the NPC knows more than you do about this situation.
Other clear signs that a GM is railroading you include that dice have stopped being rolled to determine random effects, or the GM stops referring to notes / adventure sheets, as they already know what happens next.
Most GMs can get flustered by their players when they start acting too quickly, because they need time to catch up with book-keeping (both statistically or with pages of text). Conversely, the Railroader already knows what is going to happen to the players regardless of what they do. A Railroader only gets flustered and annoyed when players insist on not following a carefully laid trail of breadcrumbs, leading to Rocks fall, everyone dies.