Bethan Roper was killed on a Christmas shopping trip when she leaned out of train window and was hit by a tree.
The 28 year old was on her way home to South Wales with friends and died in front of their eyes in the 85mph train horror.
Charity worker Bethan travelled from her home in Penarth, to Bath's Christmas market with half-a-dozen friends for their annual shopping jaunt.
She suffered serious head injuries while travelling between Bath and Keynsham. Friends battled to save her - but was pronounced dead at Bristol Temple Meads station.
A Rail Accident Investigation Branch report found a yellow sign by the window said 'Caution' and 'Do not lean out of window when train is moving'.
The window was in a door and had been fitted so it could open and allow passengers to reach the handle on the other side.
Miss Roper was on board the London Paddington to Exeter service while returning to Penarth, South Wales from a Christmas shopping trip with friends.
The incident happened at 10.04pm and she was pronounced dead at Bristol Temple Meads station shortly after 10.10pm.
Miss Roper worked for the Welsh Refugee Council charity and was chairman of Young Socialists Cardiff.
Her father, Adrian Roper, 63, released a statement after her death saying his daughter 'enjoyed life to the full whilst working tirelessly for a better world'.
The RAIB said its full investigation would examine the measures in place to 'control the risks from persons leaning out of train windows, including the threat from vegetation'.
Speaking at the time, a passenger on board said: 'Everyone in the carriage heard a loud bang and the train came to sudden halt.
'I turned and saw her on the floor, I could see straight away that she had suffered dreadful injuries.
'There were two medics on board and they gave her CPR but it was futile. The train was travelling at full speed when the accident happened.
'She was with a group of girl friends who were in a state of shock and didn't realise the severity of the situation to start with.'
A spokesman for BTP said a day after the incident: 'We were called at around 10.10pm following a report a woman had received serious head injuries while travelling on a train between Bath and Keynsham.
'Officers from British Transport Police attended along with colleagues from Avon and Somerset Police and South Western Ambulance Service, but despite their best efforts the woman died at the scene.
'Our investigation remains at an early stage, but initial inquiries suggest the woman may have been leaning out of a window when she suffered a blow to the head.'
A full inquest will be held at a later date.