Platform One was an observation space station designed as an observation deck for various events. In 5,000,000,000 it was orbiting Earth and some of the richest and most powerful people in the Universe gathered there to witness the destruction of Earth by the expanding Sun. It was moved between different artistic events. It was an Alpha Class space station, meaning it was 'top of the range'. The space station was controlled by a computer, who in turn was controlled by "the corporation".
Platform One was capable of having docked several shuttles, and used Sun Filters to shield it from the heat of the Sun and featured a force field to protect it from the exploding Earth. The use of weapons, teleportation or religion was forbidden on the platform — at least teleportation was done so under the Peace Treaty 5.4/cup/16.
It was staffed by "the steward" and a large group of maintenance staff. The platform also had plumbers to look after the air ducts, including Raffalo. The shafts also carried hot water and depending on the area of the blockage a suite could left without hot water.
It also had a suite called the Manchester Suite.
The ninth incarnation of the Doctor took Rose Tyler to Platform One on her first trip in the TARDIS.
During the Earth's final minutes, the station was sabotaged by Cassandra, as she used metal spiders to disable most of the Sun Filters. When she was caught out, she again sabotaged the station, destroying the platform's shields. The Doctor managed to turn on the force field, and save most of those on board.