Three men have been arrested during terror raids in London and the East Midlands on suspicion of supplying forged documents, Scotland Yard said.
The suspects, aged 28, 30 and 33, were taken to a central London police station.
A 33-year-old man arrested at a separate address in south-east London on suspicion of being concerned in the commission, preparation or instigation of acts of terrorism remains in custody.
While police were given until December 14 to question a further five people arrested in connection with the investigation.
They say the new rail network will reduce journey times between Nottingham, Toton, Derby and London considerably.