It shows him sitting several seats away from the girls on the South West Trains service from Richmond.
As he gets up to leave the carriage when the train pulls into St Margaret's station, he looks at them. He then reaches into his bag to pull out a small black canister and sprays CS gas in their direction.
For a moment he steps out of the carriage but seconds later returns to spray the substance directly in their faces.
Crime Action Team Pc Henrik Gottler said: "One of the girls required hospital treatment after being overcome by the fumes and complaining of irritation to her face and eyes. This would seem to be a completely random and unprovoked attack on two girls who were minding their own business.
"The CCTV footage is fairly clear and we're hopeful that a member of the public may recognise the man."
There has been at least one previous gas attack on a train in London. Last July, a gang of teenagers was caught on film spraying what was believed to be CS gas into a packed commuter train at Penge East station.
It also emerged last year that gangs are using the £10 CS sprays in attacks instead of knives or guns to escape tough sentences if they are caught and convicted.
The St Margaret's station attack was at 10.40am on 21 November. Anyone who recognises the man should contact British Transport Police on 0800 405040.
