[[quoteright:350:http://static.tvtropes.org/pmwiki/pub/images/derailed_train_455.jpg]]
[[caption-width-right:350:No doubt the train of thought of every \\
passenger aboard that train was derailed.]]

When one character becomes so sidetracked by one train of thought that he completely forgets about the original topic.

Characters prone to this include the CloudCuckooLander and TheDitz. The OnlySaneMan (who is often subject to this) can respond with a facepalm, some form of internal monologue expressing his disbelief, or an attempt to remind the invoker of this trope what they were talking about in the first place. Occasionally, the subject material brought up in the DerailedTrainOfThought can result in the recipient receiving TooMuchInformation. Responses may include the SpitTake, and for added humor, the invoker of the DerailedTrainOfThought may remain unaware of the reaction he/she has caused.

Sometimes, the invoker of this trope may not be a CloudCuckooLander, but is extremely embarrassed and, as such, wants to turn the subject to something different. Or it could be a CloudCuckooLander who also happens to be extremely embarrassed.

Compare also with NonSequitur, in which it is heavily implied that the character ''should'' be discussing one thing, but one of them goes completely against those expectations. In this trope, it is made clear by an initial statement what the conversation is about, and then one character turns the topic to something completely different. Observe:

In NonSequitur (taken from their page):

->''What we need is some way to stay in contact over long distances. Some sort of communication or "telephonic" device which is "mobile"... Of course! That's it! We'll train messenger pigeons!''

In DerailedTrainOfThought:

->''Alice: We'll need something to stay in contact over long distances. I think a cell phone should work, how about you?''
->''Bob: That would work. Say, have you ever thought about how a cell phone and a cell could be related? Maybe the cells in our body have little cell phones implanted in their DNA. Speaking about DNA, have you...''

Notice that in the second example, Alice specifically stated that they were going to talk about ''cell phones,'' whereas in the first, it was merely implied.

Related to WikiWalk and SeinfeldianConversation, in which the conversation is pointless but not necessarily unrelated.

!!Examples

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[[folder: Advertising ]]

* [[http://www.youtube.com/watch?v=cAwUUgnBhCU This Series of Videos]] by Microsoft's Bing Search engine.

[[/folder]]

[[folder: Anime and Manga ]]

* ''Manga/AzumangaDaioh'': Osaka, Cloud Cuckoo Lander extraordinaire, does this at least once, with a discussion on japanese characters that starts with hemorrhoids and goes on to the proper way to write kanji, and then to sea life.
* In the ''Manga/{{Bleach}}'' anime, Orihime manages to produce the epic [[MemeticMutation Leekspin]] meme in an Epic Tangent resulting from a crash-in with Ichigo in their school hallway. He asks if she's okay, this results from her embarrassment of bumping into him.
* In ''Manga/FullmetalAlchemist'', Ed would rather recite the periodic table than talk about his potential love for Winry.
* ''Everyone'' who tries to talk to [[LightNovel/{{Baccano}} Isaac and Miria]]. ''' ''Everyone''.'''

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[[folder: Comics ]]

* In ''ComicStrip/CalvinAndHobbes'', Calvin often goes off into these. Watterson, apparently, based Calvin's tangents off of his own wife.

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[[folder: Film ]]

* Played with in the 4th ''Film/LethalWeapon'' movie, where Chris Rock and Joe Pesci go off on a huge tangent about cell phones, for no apparent reason.
* Happens briefly to Lightning [=McQueen=] at the beginning of ''WesternAnimation/{{Cars}}'' when pysching himself up for the race.
--> '''Lightning''': I eat losers for breakfast... Should I have had breakfast?
* In ''Film/InglouriousBasterds'', [[MagnificentBastard Colonel Hans Landa]], in the middle of a conversation that will alter the course of history, pauses to ask his native English-speaking captives if he is properly using the expression "Bingo!"
* [[FirstPersonSmartass Harry Lockhart]] does this in ''Film/KissKissBangBang'', to much hilarity.

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[[folder: Literature ]]

* Luna Lovegood of ''Literature/HarryPotter'' often manages to turn a conversation into something about the magical equivalent of Bigfoot and the Loch Ness Monster. Many of the conversations she starts begins with her saying something completely ordinary, and then turning it into this trope.
* Creator/NealStephenson's ''Literature/{{Anathem}}'' seems to do this several times, but [[ChekhovsArmory every single one of them]] turns out to be a ChekhovsGun of some sort.
* The narrator of {{Mark Twain}}'s "The Celebrated Jumping Frog of Calaveras County" asks an old man about someone named Leonidas W. Smiley. The old man says he has never heard of anyone named Leonidas W. Smiley, but he ''has'' heard of someone named Jim Smiley. The rest of the story comprises a RamblingOldManMonologue about Jim Smiley and his jumping frog.
* [[Literature/LordPeterWimsey The Dowager Duchess of Denver]] tends to change the subject four or five times - in rapid succession - whenever she opens her mouth.
* In ''Literature/DaveBarrySleptHere'', Thomas Jefferson, who is writing the Declaration of Independence in an all-nighter, lets the document's subject wander to people flushing inappropriate objects down toilets, among other things.
* [[http://en.wikipedia.org/wiki/Konstanty_Ildefons_Galczynski One Polish poet]] once wrote a cycle of micro-plays and other stories. One of them - [[http://galczynski.kulturalna.com/a-6865.html that long]] - includes such a verse: "UnaccustomedAsIAmToPublicSpeaking, let me say, that... (''here happens [[SesquipedalianLoquaciousness a two-hour-long]] [[MotorMouth volcanic eruption of speech]], which is completely unrelated to the topic'')"
* Literature/CiaphasCain ('''Hero of the Imperium''')'s stream-of-consciousness memoirs occasionally drift off topic and he brings himself back to the point with "But I digress."
* [[spoiler: The ghost of]] Cyril Pennyfeather does this in his conversation with Lori and Dimity in ''[[Literature/AuntDimity Aunt Dimity Goes West]]''. Cyril is explaining how he calmed Lori and Dimity while going unnoticed by either of them:
--> "Good grief," I said softly. "You made my nightmare go away."
--> ''It would be more accurate to say that I created an atmosphere of tranquility and security in which you found it easier to sleep, and sleep, saith the Bard, is the balm of hurt minds, great nature's second course, chief nourisher in life's feast. Macbeth Act two, Scene two. [[LampshadeHanging But I digress.]]''
* Literature/TristramShandy is made of this. The book is about the supposedly interesting life and opinions of Tristram Shandy, but the narrator keeps getting sidetracked, not even getting to Tristram's birth before partway into the second volume.

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[[folder: Live Action TV ]]

* In ''Series/{{Seinfeld}}'', George's boss, Mr. Steinbrener, always gets sidetracked by one thing or another when George tries to talk to him.
* The Doctor, on ''Series/DoctorWho''.
-->''This is my timey-wimey detector. It goes ding when there's stuff. Also, it can boil an egg at thirty paces... whether you want it to or not, actually, so I've learned to stay away from hens. It's not pretty when they blow.''
* The Swedish comedy show ''Hipp Hipp'' had Melodikrysset, a musical crossword aired on radio, wherein the host would play a song and then explain whet you were supposed to write. After playing In the summertime the explanation is as follows.
--> '''The Host''':That was In the summertime by Mumbo Jerry and it's about the summer, in the summer people like having ice-cream. Personally I'm fond of 88:an. 88:an has crushed nuts in it so therefore [[GroinAttack Pungspark]] goes on vertical no. 13.
* ''Series/TheNewsroom'' features this regularly, most notably when a meeting where a discussion regarding the possibility of American troops having committed a war crime segues into a long discussion about whether Santa Claus has eight or nine reindeer.
* The ''Series/MontyPythonsFlyingCircus'' episode "Erizabeth L" has a police inspector (from the Film Fraud Division) who's apparently incapable of arresting a dangerous criminal who's impersonating famous directors without digressing into a lengthy biography of the director in question.
* Sophia did this practically every time she told a Sicily story on ''Series/TheGoldenGirls''. She'd get off on a tangent, then say "But I digress."

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[[folder: Music ]]

* The Music/WeirdAlYankovic song "Albuquerque" is a long, rambling screed that starts off with an account of his mom serving him sauerkraut for breakfast that touches on the number of molecules in Creator/LeonardNimoy's butt, moving out of the house, playful shriners, fluffy towels, ukulele-playing lepers, back-shavers, Albanian women with really bad body odor, Doctor Pepper, an airplane crash, an overly stuffed suitcase, eating soup out of ashtrays, air conditioning, those little mints that housekeepers leave on your pillow at high-class hotels, hermaphrodites with that Flock-of-Seagulls haircut, weasels, peaches, [[TheLegendOfZelda Zelda]], mint-flavored dental floss, working at the Sizzler, excessive ear wax, Torso Boy, and a lucky autographed glow-in-the-dark snorkel, all to end up at the entire point of the song:
-->''I know it's kinda been a roundabout way of saying it, but I guess the whole point I'm tryin' to make here is '''''I HATE SAUERKRAUT!'''''"''

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[[folder: Real Life ]]

* According to the man himself, author Creator/PeterDavid is so prone to doing this that he called his column in ''Comics Buyer's Guide'' "But I Digress..." as a [[LampshadeHanging lampshading]] of it.

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[[folder: Western Animation ]]

* Many of SethMacFarlane's works are prone to this. ''WesternAnimation/FamilyGuy'' and ''WesternAnimation/TheClevelandShow'' employ [[CutawayGag 10-second shorts to provide comedy]], connected only to the dialogue by a "Just Like X" statement made by one of the characters.
* ''WesternAnimation/InvaderZim'' - Don't even ''try'' to have a coherent conversation with [[CloudCuckoolander Gir]].