Corporal Ismaila Ceesay, a 28-year-old information management specialist from Stratford, east London, said part of the exercise was arriving in civilian clothes and changing into uniform only once through the secure barriers.
He said: “I’ve reached into my London roots and adopted a London look to blend in like a local, so no-one can suspect I’m anything but a commuter going to work.
“I’ve got my hoodie on, changed my gait and I try to blend in.”
His family thought he was on leave.
He said working in the disused Tube tunnels was “like being in an episode of Stranger Things, with the red lights and the dark shadows and the fact that nothing seems quite as it should be”.