Bethan Roper, 28, died after hitting her head on an overhanging tree branch whilst leaning out of a Great Western Railway (GWR) train window in 2018.
The train was travelling at around 75mph near Twerton in Bath at the time of the incident.
The line, which runs from London Paddington to Exeter, was using carriages with droplight windows which enabled passengers to use the handle on the outside to open the doors at platforms.
Ms Roper, from Penarth in Wales, worked for the Welsh Refugee Council, chaired the Cardiff West branch of Socialist Party Wales, and was a Unite union convener.
She was returning from a day out Christmas shopping in Bath and was intoxicated when she leant out of the moving train on 1 December, 2018.
GWR have been fined £1million over the incident. It was ordered to pay a further £78,000 in costs after pleading guilty to two counts of breaching health and safety law.
The rail safety watchdog’s chief inspector called Ms Roper’s death “a preventable tragedy”.
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In 2016, a passenger died in a similar incident near Balham, south London.
Ms Roper’s death followed GWR’s failure to implement safety recommendations regarding the windows on such high speed trains.
“Although GWR was already aware of a number of previous incidents, the company did not produce a written risk assessment for droplight windows until September 2017,” the regulator said.
ORR said they wrote to GWR to notify them of the shortcomings of their assessment, but said that it was not revised.
The actions identified to reduce such risks were not implemented before the incident involving Ms Roper in 2018, according to the ORR.
Richard Hines, ORR’s Chief Inspector of Railways, said their thoughts remain with Bethan Roper’s friends and family.
“Her death was a preventable tragedy that highlights the need for train operators to proactively manage risks and act swiftly when safety recommendations are made to keep their passengers safe.
“Our investigation found that GWR fell short in its responsibilities, and this prosecution reflects the serious consequences of that failure. We welcome the actions taken since by GWR and the wider industry to reduce the risks. Safety must always remain the first priority across Britain’s railways.”
A GWR spokesman said that the death of Ms Roper was a “tragic accident” and they “remain committed to continuously improving passenger and colleague safety” across the network.
All trains with droplight windows have since either been withdrawn from service or fitted with controls to prevent windows being opened while trains are moving.