The Galaxy class variant was an experimental type of starship classed under different designations, such as the "W13 Galaxy class" and the Galaxy III class. This class also included a 2395 version of the USS Enterprise-D in Q's anti-time future (TNG: "All Good Things...").
Galaxy III classEdit
The Galaxy III class was brought into service in 2377 the first vessel being the USS Excelsior. These ships were very powerful, highly advanced dreadnought types, capable of supporting a crew compliment of 1,200. Galaxy III vessels used data storage networks featuring bio-neural gel packs and subprocessor nodes however the systems could be easily downgraded to isolinear circuitry when needed. (Star Trek: Hidden Frontier)
W13 Galaxy classEdit
The W13 Galaxy class was created in the form of the USS Xena in the early 2370s. Its specs were largely based on Jean-Luc Picard's description of a future version of the USS Enterprise-D he had once encountered.
Though Starfleet attempted to jump the gun and create a version of the Galaxy-class like the one in the future, they inevitably were unsuccessful in making it that much more advanced. A third nacelle was one of the big differences, but didn't make the ship go past warp 9.
It did, however, enable the ship to maintain an even more stable warp shell that could withstand more natural spacial phenomenon.
The vessel was launched as the Xena, with increased medical facilities to service injured people. It eventually became a vessel to assist other vessels in after-battle situations. After the creation of the Xena, in the early 2370s, two more starships were built to this class. (Star Trek: Phoenix-X)
Star Trek OnlineEdit
The Galaxy X class was a refit of the Galaxy class following the Klingon Empire renouncing the Khitomer Accords in 2399. The class included an extra nacelle as well as been equipped with a cloaking device. It was heavily armed and equipped with a power spinal lance phaser. (Star Trek Online)