Naritaverse

The Naritaverse is the unofficial name for the universe in which Baccano! is set alongside Durarara!!, Vamp!, and Etsusa Bridge, three other light novel series written by Ryohgo Narita.

Durarara!!
Set in Tokyo's Ikebukuro district in the early 2000s, Durarara!! follows the stories of many characters and gangs of varyingly dangerous and eccentric personalities as they comingle and cause chaos together. Some of these individuals and groups include: a dullahan courier, a manipulative information broker, the strongest man in Ikebukuro, the Russian Mafia, the internet-based group Dollars, the Yellow Scarves gang, and an underground doctor.

Aside from Baccano!, Durarara!! is the only other one of Narita's light novel series to have received an anime adaptation. It also has a manga adaptation and two videogame adaptations. Durarara!! is illustrated by Suzuhito Yasuda.

Vamp!
Also set in the early 2000s (2004/2005), Vamp! takes place on the fictional island Growerth off the coast of Northern Germany and tells the story of humans, vampires, werewolves, and other supernatural creatures with imaginative zeal. Especially of interest are the vampire twins who live in Waldstein Castle along with their adoptive father and other supernatural creatures, a dhampyr mayor with a petty personality, the odd beings known as Eaters and the large and colorful cast of vampires who make up The Organization....

Etsusa Bridge
Set in the late 2010s (2014, 2019-2020), Etsusa Bridge revolves around a gigantic, incomplete bridge connecting the Sado and Niigata islands and the nameless artificial floating city at its center. The city and bridge have become a haven for criminals, outcasts, and mafia outfits to make a dangerous life for themselves.

Light novels

 * Ronny Schiatto is apparently friends with Celty Sturluson, a dullahan who works as an underground courier in Ikebukuro. The details of how they met and came to know one another are unknown.
 * The Unnamed Photographer who narrates The Rolling Bootlegs lives next door to high school student Mikado Ryūgamine in Ikebukuro.
 * In August 2002, Bride says that he'll have to go see "Orihara" to return the church key and thank him for agreeing to take care of corpse disposal for Bride's group SAMPLE. This 'Orihara' is one Izaya Orihara, an information broker who is based in Shinjuku.
 * While on vacation in Ikebukuro, Nile apparently met and got into a fight with Shizuo Heiwajima, who came out of the fight on top.
 * American filmmaker John Drox has hired Kasuka Heiwajima to act in several of his movies.
 * Volume Four of Durarara!! compares the serial killer Hollywood (who dresses up in monster costumes while killing people) to Isaac & Miria, two thieves who are famous for having committed multiple robberies dressed up in all sorts of costumes in the past.
 * In Volume Eight of Durarara!!, Saki remarks that Charon Walken and Kasuka are strkingly similar to one another.

Anime

 * In Episode 11 of Durarara!!, Isaac and Miria show up for the first assemblage of the Dollars in Ikebukuro.
 * In Episode 12.5, Isaac and Miria discuss what they believe to be hitodama (or 'ghostly fireballs') spotted in the night sky. These are actually people covered in glow paint that were thrown by Shizuo.
 * Throughout the Durarara!! anime adaption, clips from the 2007 anime adaptation from Baccano! play on outdoor screens in the city (near the movie theatre especially), and posters featuring Baccano! characters can also be spotted here and there. Apparently, these are actually promotional material for a film (directed by John Drox) that is based off the recently uncovered events that took place aboard the Flying Pussyfoot during 30-31 December 1931.
 * In episode 11 of Durarara!! x2 Ketsu (so episode 35 of season 2), Mikado mentions that his fist gun is a weapon that a "terrorist in America" designed many years ago. The terrorist in question is actually Huey Laforet, and the gun is the same type of gun that Goose Perkins uses aboard the Flying Pussyfoot.

Baccano! and Vamp!

 * Carnald Strassburg (a famous historical figure from Growerth) is credited by Carla Alvarez Santonia in 1711: Whitesmile as the engineer who designed House Dormentaire's floating barricade of ships stationed in Lotto Valentino's harbor by 1711. In the same novel, Lucrezia de Dormentaire also mentions him as the man responsible for Growerth's perfect sewage facilities.
 * Several of his paintings also hang in Nebula's headquarters in Chicago in 1934.
 * The Advena Avis is procured at great debt by Dalton Strauss from the Mars Family, a vampire clan that at the time was powerful and teeming with members. (By the early 2000s, the Mars Family has lost all its members save for its leader, Romy Mars. It is possible that Dalton contacted Romy herself to make the transaction).
 * According to Claire Stanfield, one of his colleagues back in the circus was a self-professed vampire who could perform such tricks as turning his hat into a flock of bats. The vampire's shining suit, coupled with the specific mention of his name by Charkie, pinpoint him as Doubs Hewley, a vampire from Vamp!.
 * In Vamp! IV, vampire Rude Gardastance stares at the Nebula skyscraper Mist Wall from his own building in the early 2000s. The Mist Wall is a highly important location in Baccano!. He also mentions that he has a debt to Nebula's chairman from five generations - it is possible that this chairman is or has some relation to Karl Muybridge.
 * Rude is also referenced in 1934: Peter Pan in Chains, when Chairman Muybridge remarks that Ladd Russo should have gone to "Rude, Beriam, or one of those other fellows with money" if he'd wanted to cause trouble for Nebula back when he was a youngster.
 * Ending 21 of the Baccano! NDS Game essentially serves as an advertisement for Vamp!. It features Isaac and Miria aboard the Flying Pussyfoot, where they come across a moving mass of red liquid which forms itself into letters. They are convinced that there is an invisible man vampire in the train and flee. The mass of red liquid (blood) is actually the vampire Gerhardt von Waldstein. This encounter most certainly did not actually happen.
 * In 1710: Crack Flag, The Third Library is said to be funded by a "nobleman from an island in northern Prussia." It is possible that this island is actually Growerth, the island upon which Vamp! is set.

Baccano! and Etsusa Bridge
It should be noted that the powerful Nebula corporation has influence in all the series, especially Durarara!! and Baccano!.
 * In volume three of Etsusa Bridge, serial killer Yakumo Amagiri says that he has heard of a female assassin in the United States who fought with two katanas. He is thinking of Maria Barcelito.
 * Baccano! character Angelo states that he has a son called Carlos (who turns three in 2002) whom he has not met yet. Angelo is a Spaniard, an excellent sniper/gunman, and wears blue shades. In Etsusa Bridge, there is a young man called Carlos who is also Spanish, an excellent sniper/gunman, and wears blue shades. It is very possible that these two Carloses are in fact the same person.