He said his head was burned by a "really hot intense fireball above my head" and added: "There were people a lot worse than me".
The explosion at the Parsons Green Station in southern London occurred at about 8:20 a.m. local time - the height of rush hour on a busy commuter line into central London.
Pictures and video posted on social media showed a flaming bucket - which is said to have had wires coming from it - inside a Lidl carrier bag on the floor of a carriage.
The incident comes as tensions are high in London, which has been struck repeatedly by extremist attacks this year.
London Underground blast: People leave the scene of an explosion at a southwest London subway station in London on Friday.
Eyewitnesses outside the station reported seeing people with facial injuries. The London Ambulance Service says none of the injuries are thought to be life-threatening. The country's threat level now stands at "severe", the second-highest behind "critical".
US President Donald Trump denounced the attack by "sick and demented people who were in the sights of Scotland Yard". Passengers described seeing a wall of fire rushing through the train vehicle. Counter-terrorism police are also on the scene.