A 14 year-old girl was 'flashed' travelling home on a train after visiting the poppy display at the Tower of London with her mother.
The incident happened on the 4pm Liverpool Street to Shenfield train on Saturday, 8 November.
Detectives have released a photograph of a man they would like to talk to in connection with the investigation.
A passenger who tried to stop 5 men harassing a woman who was travelling alone on a train has been attacked and left with a broken nose.
Police have released CCTV images of five men they are trying to find.
Transport for London is to take over the running of some mainline rail services and stations.
From around 2015, TfL will be responsible for some West Anglia routes from Liverpool Street station in London to Enfield Town, Cheshunt (via Seven Sisters) and Chingford.
The route includes 25 stations, with TfL taking over 23 of them. Liverpool Street and Chingford stations will remain the responsibility of Network Rail or Greater Anglia.
Improvements made by TfL will include improved reliability and security and better passenger information.
London Mayor Boris Johnson said today: "You only have to look at the success of London Overground to see how TfL taking over this commuter service will transform the journey for passengers.
"It will also open up the north-east of London and drive jobs and growth in Tottenham and beyond."