A woman suffered fatal head injuries while leaning out of a train window below a warning sticker, an accident report has found.
Bethan Roper, 28, died on the Great Western Railway train near Twerton in Bath, England on December 1.
Her head was out of the window of a door when it was struck by a tree branch while the train travelled at 85mph.
A preliminary report by the Rail Accident Investigation Branch (RAIB) found that a yellow "Caution" label above the door said "Do not lean out of window when train is moving".
The door was fitted with an opening window to enable passengers to access the handle, which was on the outside.
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".
In August 2016, railway fanatic Simon Brown, 24, was killed while leaning out of the window of a Gatwick Express train near Balham, south London. His head hit a signal gantry at 61mph.
In its report, the RAIB noted that a yellow warning sticker on the door was in a "cluttered environment" among other information signs, and recommended that train companies should do more to stop passengers from putting body parts out of windows.
In November, public sector company Network Rail was asked by rail minister Andrew Jones to set out a plan for how it will do more to protect biodiversity during tree felling.
This followed criticism over the amount of trees being cut down in some areas.