|WARNING! This article contains MAJOR spoilers for the recently released novel Uncertain Logic. Caution is advised.|
In the mid-22nd century, Tobin Dax was recruited by the Cochrane Institute of Alpha Centauri to participate in the testing of a prototype transporter. (DS9 - The Lives of Dax short story: "Dead Man's Hand")
In 2157, Dax, Underhill, and Skon were present at a failed test of countermeasures to the Romulan remote-hijack system. Dax and Underhill participated in a successful test later that day after Skon had been removed for being a security risk. (ENT - The Romulan War novel: To Brave the Storm)
Following the war Dax worked on several conceptual designs for what unified Starfleet vessels would look like. Dax felt that Earth's starship designs were the most logical for a fleet of exploration, and believed that even with the combined technologies of the Federation members the ships would look largely like United Earth Starfleet ships such as the NX-class, Columbia-class or the Intrepid-class.
In 2163, Dax requested a posting on the USS Pioneer after he was asked to consult with the chief engineer following the installation of an upgraded warp core, one capable of sending the Intrepid-class up to warp 5.6. He was assigned as chief engineer after Alan Sheehan was forced to undertake a leave of absence for medical reasons.
While on the Pioneer, Dax' joined nature was discovered by the ship's chief medical officer, Doctor Therese Liao. At the time the joined nature of the Trill was not widely known, and Liao respected Dax's wishes to keep that nature a secret. She did suggest to Dax that he confide in Captain Reed, which he eventually did. Reed also kept Dax's nature secret. (ENT - Rise of the Federation novel: A Choice of Futures)
Spoiler warning: Plot and/or ending details follow.
In 2165, the Ware attacked the Pioneer, severely damaging her. His assistant chief engineer, Lieutenant Prentis Morrow, died in the encounter, which reminded Dax how dangerous space travel can be, and he decided to leave Pioneer. He visited his friends, Skon and T'Rama, on Vulcan, who invited him to stay in their home as a guest, until he decided what he wanted to do next.
While on Vulcan, Tobin met Iloja of Prim, the exiled Cardassian poet who was living on the planet at the time. During his stay, Tobin often visited debate houses with his friends, including Iloja and Phlox. He followed the events of the unfolding political situation following the claim that the Kir'Shara was a forgery created in the previous decade by then Captain Jonathan Archer and the United Earth Starfleet to overthrow the Vulcan High Command. He was also instrumental in preventing the destruction of Vulcan Space Central by discovering the sabotage of the docked USS Endeavour's warp engine. After the crisis was resolved, and having spent much time with Skon and T'Rama who just had their child, Sarek, he decided to build a family of his own. (ENT - Rise of the Federation novel: Uncertain Logic)
Although shy, Tobin enjoyed magic tricks. During the Earth-Romulan War, he demonstrated his abilities to Skon, who was not particularly impressed. (DS9 - The Lives of Dax short story: "Dead Man's Hand")
Tobin started a family sometime after 2165. His child Raifi was known to have put him "through hell." (ENT - Rise of the Federation novel: A Choice of Futures; DS9 episode: "Nor the Battle to the Strong")