A commuter and a TfL employee were attacked by a man who threw a substance thought to be acid in their faces while on the London Overground.
The female passenger and the male worker were attacked at South Tottenham, north London.
Fellow travellers poured water on the pair, who were treated by the London Ambulance Service's Hazadous Area Response team on the scene, before being taken to hospital. They have since been released.
British Transport Police were called to the incident just before 8pm and are now hunting the suspect.
A spokesman said: “We were called to South Tottenham station at 7.57pm on Wednesday (24 June) to a report of a member of staff and a passenger having had a substance thrown over them whilst on a London Overground train.
“This did not result in any serious injuries although both the male staff member and the female passenger were taken to a local hospital as a precautionary measure and have since been released.
They are also seeking to identify the substance, although eyewitnesses described it as acid.
Anyone with information is asked to contact BTP on 0800 40 50 40 quoting reference 566 of 24/06/155.