"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 small amount of railroading occurs in nearly all campaigns and is considered reasonable and necessary for a satisfying story, 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.
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.
It should be pointed out that some degree of railroading is a necessary requirement when playing pen & paper RPGs, mostly because there is only so much preparation that can be done, and that an adventure usually has a pre-defined start and end point... so if the players cannot be bothered slaying the arch-demon and decide to go kill the pirate king instead, then they need to be steered back on the path that keeps them within what the GM has available.
A good GM knows that an adventure is not a straight line from A-B, but rather a wavy zig-zag line that keeps things interesting, and so just makes shit up to accommodate the players decisions, rather than making their decisions in advance.