Derailing tends to happen on Fora and Blogs the most, but it's generally something that can happen anywhere there is a discussion of any sort. It is simply the unwanted and forced change of a subject to one that the derailer wishes to discuss, or one with which the derailer wishes to start a Flame War in hopes of getting the original topic forgotten.
Derailing differs from simple Trolling in that where the troll wants to start a fight or cause damage, consequences be damned, the consequence of the topic being changed is the goal of someone engaged in derailing. This often can make methods of combating trolls (such as the application of whole thread locking and deletion or the banning of trolls and others behaving badly) worthless at best and in some cases extremely counterproductive in an attempt to deal with a derailing attempt.
Discussions that are Serious Business or reference Serious Business tend to attract derailing the most, especially anything referencing a Real Life issue or incident or discussing Unfortunate Implications. Common methods of derailing often include victim blaming, various forms of Ad Hominem including but not limited to poisoning the well and direct Ad Hominem, raising an entirely separate concern that is at most only loosely related to the subject under discussion (e.g. a tax rant on a thread discussing response times for the fire department or a rant about fathers' rights in a feminist issues discussion), Misplaced Nationalism, and outright Trolling. Comparing someone's argument to Nazis is akin blowing up a railway bridge, dropping the train into the sea and then pissing on the remains in derailing terms.
Another common method of derailing is threats, which if believed can chill a discussion fast because no one wants to get hurt or sued, and even if not believed can make a discussion instantly about the threatening person rather than what it was originally about. This is why (among other reasons) many communications venues take a very hard line on making threats (for example, on The Other Wiki, making a legal threat is in and of itself grounds for an immediate ban, and in most places threatening physical harm in a valid way is grounds for not only a ban but for law enforcement to be notified.)
Finally, derailing must be intentional, if it is not intentional, it is simply the Left-Fielder. This is the "forced" part of it: if, say, a general discussion on pit bulls turns into a fight about how dangerous pit bulls are, it wasn't derailed, since that was an arguable natural outcome of the discussion, even if a bad or unwanted one. A discussion about training pit bulls turning into a raging Flame War over whether one can breed a pit bull with a Chihuahua due to one poster posting graphic pictures of the mating, however, is very much likely so due to derailing. As another hypothetical example, mentioning Adolf Hitler in a discussion about neo-Nazis is not derailing. Comparing your opponents in a discussion about an MMORPG to Adolf Hitler, on the other hand, is.
Compare G.I.F.T. and Troll, compare Garbage Post Kid which is occasionally an extreme example of derailing, and compare Godwin's Law, which is almost always an instant derail of any discussion in which it is an Invoked Trope. See Serious Business, and Flame War. Derailing is a favorite tactic of The Shill and those engaged in the use of AstroTurf and Polish the Turd, and it is actually advice sometimes given to deal with an Internet Counterattack. Related to Griefer and Troll in that the derailer, the griefer, and the troll all seek disruption, albeit, as mentioned, in different contexts for different goals.
(The best way to tell the difference is to ask oneself what the goal seems to be. If there seems to be no intent at all, it is the Left-Fielder. If the intent is enraging everyone in range, it's likely a Troll, or if in a game, a Griefer. If the intent is to shut someone or everyone up or change all discussion to the new topic, it is likely derailing.)
Compare and contrast Change the Uncomfortable Subject, where someone steers the discussion away from a topic they find embarrassing or upsetting, without the disruptive motivations typical of this trope.
Not to be confused with actual trains coming off actual rails.
- Derailing for Dummies is a sarcastic parody "guide for derailers" in discussions about social justice issues. It lists some of the most common derailing tactics in action for those interested in keeping discussions/communities on topic and stopping derail attempts.
- A small scale meme similar to this is often used as an anti-Trolling tactic on GameFAQs. If a poster makes an obvious troll topic, a topic that is still a sore spot for a community, or a topic that has been overdone, another poster will say something along the lines of "This topic sucks. Its now about trains." Cue the other posters discussing everything they know about the railroad industry.
- A humorous example emerged from The Jedi Council Forums. An otherwise regular forum debate brought up Waru, and suddenly within hours, a board-wide Church was formed.
- As a combination of this and AstroTurf, the "Internet Water Army," which uses trolling and derailing as an AstroTurf tactic. Note that while the Water Army may be exclusive to China and/or Chinese issues, any Real Life Mega-Corp, political party, or even religion with enough devotees or money could manage the same thing.
- This is discussed in Seinfeld after Susan, George's fiance, asks to "change the subject" when he's ranting about bathroom stall doors leaving your feet exposed. George and Jerry both agree that changing the subject is impolite and unnatural, as the topic of discussion should resolve itself based on its own momentum.