British Transport Police (BTP) said the incident happened at the Gipsy Lane level crossing, Needham Market, near Ipswich at about 14:00 BST.
The woman was struck by the Liverpool Street to Norwich service, officers said.
Paramedics from the East of England Ambulance Service were called to the scene but the woman was pronounced dead.
A BTP spokesperson said: "Officers are currently on scene working to establish the identity of the deceased and inform her next of kin.
"The incident is currently being treated as non-suspicious and a file will be prepared for the coroner."

Dr Ben Clark, 40, was one of three air ambulance staff to be honoured at the Pride of Britain awards for treating victims of the Alton Towers Smiler crash in June.
Former England captain Beckham decided to join in when Dr Clark was photographed at Monday's ceremony.
The Midlands Air Ambulance Service later tweeted the photo.
Beckham was among celebrities attending the event at London's Grosvenor House, which honours brave members of the public.
Dr Clark and his colleagues previously said they "broke every rule in the book" to rescue those stuck on the ride after it crashed.
Speaking about the rescue, Dr Clark, a volunteer with North Staffordshire BASICS emergency doctors, said: "I didn't know if it was going to work and if they were going to stay alive.
"When you see people dying in front of your eyes and you know you can't do anything any quicker - it was very difficult."

Four officers were injured when the Met's Flying Squad swooped on the men in Piccadilly at 11:25 BST.
A Taser was discharged several times and the policemen were hurt as the men tried to escape on mopeds.
Four men have been arrested and are being held at London police stations.
A Met Police spokesman said three officers suffered minor injuries and the fourth is in hospital in a serious condition.
Store owner Avi Reed said the gang smashed a glass cabinet, causing more than Â£300,000 of damage,
Mr Reed said police contacted them at the Chronext store earlier to warn them that there might be a robbery, and the shop was closed to protect customers.
He said: "At about 11:30 we saw three guys, they had helmets on. One tried to smash the door with a hammer. The glass didn't break as it's protected but he managed to get in.
"Another guy had an axe and managed to smash a cabinet, while the other stayed on the motorbike. They caused enormous damage."
Mr Reed praised the actions of the "amazing" police.
Det Supt Craig Turner, said: "Our officers are absolutely committed to keeping London safe and target the criminals who pose a danger to Londoners and our city businesses day in, day out.
"They know that this work carries with it very real risks and dangers but take this in their stride as part of serving the public.
"However, today's events highlight how the reckless behaviour of those determined to evade justice, heedless of the consequences, can have a real impact on all those involved."
Piccadilly was closed eastbound at Hyde Park Corner for about six hours while investigations were carried out.
A woman has been killed on a level crossing in Suffolk.
[NEXT_CONCEPT]
A medic has been "photobombed" by David Beckham at an awards show while tucking into a sausage roll backstage.
[NEXT_CONCEPT]
A moped gang armed with knives and an axe has been intercepted by police in a "terrifying" attempt to rob a luxury watch store in central London.