Interstitial Mass Transit System The Interstitial Mass Transit System — also known synonymously as the STS after the company that maintained it — was a public transport and freight network , derived from transmat technology , that linked human s ' settlements throughout the Sol system between the late 21st and mid 22nd centuries AD . It was run by the Sol Transit System company from a central control facility located beneath Olympus Mons on Mars . ( PROSE : " Transit " ) History . By the middle of the 21st century , humanity had developed close - ended transmat technology to a degree of technical reliability , energy efficiency and cost effectiveness that it replaced traditional means of freight transfer . However , an attempted Martian invasion exposed a number of critical and systemic flaws in the T - Mat network . This and the manner in which the subsequent war was won through the mass transmat of human troops onto the Martian surface saw T - Mat replaced by an interstitial mass transit system that combined matter transmission with specially adapted trains similar — running on friction fields instead of rails or monotracks — to those of the British Underground . Significant components of the initial system infrastructure were sourced from the Yamatzi Corporation of Japan , and the Central Line was established before the turn of the 22nd century . Although newer branches were able to benefit from improvement in technologies and methodologies — such as the claimed affinity Africa n manufacturers possessed for using the transient nature of reality as a basic operating principle — older ones were more likely to be serviced than replaced . By the 2110s , the network had grown into what those familiar with it liked to call a sprawling beast . As a result the STS Special Maintenance crew enjoyed pointing out that very few people understood just exactly how the entire system worked and all of them were on that crew . While the veracity of the claim was never tested , the Chief Controller granted them a great deal of latitude in their behaviour in return for their ongoing expertise . By the end of the decade , the decision was made to open the first interstellar line . Properly called the Stellar Tunnel — and more referred to more commonly by the portmanteau " Stunnel " — it reached 40 light years between Mars and a planet in the Arcturus system . However , the quantum mechanics involved in maintaining and opening the connection allowed a hostile intelligence to punch through into this dimension from its own and infect the system . The incident resulted in the massacre of high ranking members of the Union of Solar Republics , and led to the abduction and physical mutation of several passengers throughout the network into a beach - head invasion force . Around the same time , the network ' s convoluted and complex structure spontaneously generated a self - sustaining high - functioning artificial intelligence . This AI , the Special Maintenance crew , the Seventh Doctor and Kadiatu Lethbridge - Stewart were responsible for defeating the invasion and forcing the hostile intelligence to return to its home dimension , thereby releasing Bernice Summerfield , who had been functioning as its host . ( PROSE : " Transit " ) How it functioned . Like transmats , the system relied on carrier waves transmitted between stations . At each end of a station platform was an induction system — Lorenzo attractors held in a precise pattern by a gravito - magnetic field . It required only a slow but constant rotation of the inductors to generate the multi - frequency carrier waves that connected the station nodes ; the presence of a carrier wave could be confirmed by the presence of a visual event horizon . While signals could travel along the carrier wave , physical objects were incapable of moving through ; two sets of inductors simultaneously spun at a matching harmonic while the correct amount of power was fed into the carrier wave was required to establish a sub - space tunnel through which a real object could pass . The physical dynamics of the tunnels and the dimensional translation were controlled by field regulators whose output was rated in angstroms . A device called a gateway compensator was responsible for preventing matter annihilation in the event of signals from multiple sources being simultaneously reintegrated in the real world . Trains were energy shielded as a safety precaution to prevent the torque effects of crossing the event horizon of a tunnel shredding them apart . Freesurfers — individuals using surf boards to traverse the interstitial vortices — replaced the energy shield with a complex sonic wave generated through the use of mantras , but left the surfers exposed to the psychedelic sensory input of subspace - transit . Passage through a tunnel imparted real - world momentum ; while trains had technological counter measures to this acceleration , the purpose of a freesurfer ' s partner was to chant a counter - mantra that acted as a brake . Originally locomotives had to possess generators powerful enough to generate a shield large enough to cover the entire train in a single field to enable all - at - once dimensional transition , but advances in technology led to the creation of split - section integrators that could handle carriages individually in real - time as they passed the event horizon of a tunnel . System details . A curious artefact of the System ' s design was that it ran entirely on Greenwich Mean Time , regardless of the local timezone ; the only exception was Triton , which for historical reasons ran at GMT + 5 . Structure . Freight tunnels ran parallel to a main line . Prominent stations . Arcturus Terminal , Mars . Located in the bedrock beneath the permafrost of the North Pole . " The Connection " . A series of submarine stations along the Ionian sea built for floating cities that never were , they became a multi - stop flea market and trade meet . Dentsu . A major city on Pluto . Kings Cross , London . The primary Northern European transit hub as a result of a backroom deal , the station gathered half the transcontinental feeder lines in Europe and channelled them into the next level of the network . Lowell Depot , Pluto . Terminus of the Central Line . Nagorno - Karabakh . A major city on Pluto . Yamaha . A major city on Pluto . Triton Central , Triton . Named " Mitsubishi " by the local zaibatsu as part of the Japanese colonial claims to significant tracts of the outer reaches of the sol system . Despite 30 years of civil rights legislation , by the end of the 2110s it remained one of the most ethnically segregated metropolises in the system .