The 21-year-old was arrested in Hounslow at 11.50pm on Saturday, the Met's Counter Terrorism Command said.
The development comes after the arrest of an 18-year-old man in the departure area of Dover ferry port on Saturday morning. The suspect remains in custody at a London police station.
On Saturday, a search was carried out at a house in Sunbury-on-Thames, Surrey - home to foster care couple Penelope and Ronald Jones, aged 71 and 88 respectively, who previously received MBEs for services to children and families.
The couple are said to be staying with friends for at least the next five days following the police raids, during which surrounding houses were evacuated by counter-terror officers, with residents told they had "one minute" to flee their homes.
Friend Alison Griffiths said the couple - foster parents for almost 40 years who had taken in up to 300 children, including eight refugees - had an 18-year-old and a 22-year-old staying with them recently.
She described Mr and Mrs Jones as "great pillars of the community", adding: "They do a job that not many people do."
A key strand of the investigation has focused on CCTV as officers comb through footage to establish who planted the device, and when and where it was placed on the train.
The first suspect was arrested at around 7.50am, in the port which is the busiest ferry hub in Europe and serves as a commercial gateway to the French coast, including Calais and Dunkirk.
Tourist Daniel Vaselicu, 31, said he saw the "young and light-skinned" man being interrogated by two unarmed police officers moments before his arrest in Dover.