A passenger was killed while leaning out of a train window below a warning sticker, accident investigators found.
Bethan Roper, 28, from Penarth, suffered a fatal blow to the head when she was struck by a tree branch.
The charity worker had been shopping at Bath’s Christmas market with friends on 1 December and was travelling home.
A preliminary report by the Rail Accident Investigation Branch (RAIB) found the Bristol-bound train was travelling at 85mph at the time.
The accident happened at 22:04 GMT near Twerton and Miss Roper was pronounced dead at Bristol Temple Meads station shortly after 22:10.
The older-style Great Western Railway (GWR) train’s doors were fitted with an opening window to enable passengers to access the handle on the outside.
The RAIB said that a yellow “Caution” label above the door said: “Do not lean out of window when train is moving.”
Its full investigation will examine the measures in place to “control the risks from persons leaning out of train windows, including the threat from vegetation”.
Miss Roper worked for the Welsh Refugee Council charity and was chairman of Young Socialists Cardiff.
Her father, Adrian Roper, 63, said his daughter “enjoyed life to the full whilst working tirelessly for a better world”.