Paddington Station was partially cordoned off on Friday with passengers stopped and turned around by staff who said they were dealing with a “suspected virus case”.
Photos show a woman sat alone with a small suitcase next to her, surrounded by medics and staff members wearing masks at about 11.30pm.
It raised fears the case was coronavirus but the exact identity of the illness has not been confirmed by authorities.
London Ambulance Service said two ambulances were sent to the station and two people were taken to hospital.
Mack Grenfell, 24, was on his way home from work and caught his usual train from Oxford to Paddington station when he saw the cordon.
The advertising manager said: “I was walking towards one of the exits and I noticed that Paddington was unusually empty - it’s normally quite full of people.
“I was headed toward the west exit to find something to eat and got stopped by a staff member who was blocking that half of the station off from passengers.
“The station staff told me I couldn't go through there because there was a suspected virus outbreak.”
He said they pointed to the woman, who was surrounded by a circle of about 15 people, including medics - most of whom looked to be keeping their distance.
“I wasn't allowed any closer but she seemed relatively calm despite the situation,” he added.
“I took the photo and then headed to the tube to go home as the way to the underground was still open.”
Paddington Station is open and trains appear to be running as normal.
On Saturday, it was announced that one of the two patients with confirmed coronavirus in England is a University of York student.