More than 80 people were treated in hospitals after the crash late on Friday afternoon, and 28 remained hospitalised a day later, British Transport Police Chief Constable Lucy D'Orsi said.
Buckingham Palace said King Charles III "is greatly saddened" by the crash.
It said "his thoughts and sympathies are with the family of the deceased and with all those injured or affected by such a tragic incident".
Police and accident investigators are working to understand why a commuter train bound for London's St Pancras Station slammed into the back of another train headed for the same destination on Friday afternoon.
Photos and videos posted on social media showed dozens of people, some with bandages but also many who appeared uninjured, standing and sitting among emergency vehicles parked on a road parallel to the train tracks.
Peter Knapp, one of the passengers, described being thrown forward by the impact, then seeing fellow travellers with broken bones and bloody injuries.
"People were crying, screaming. People were so scared and confused," he said.
"I got up and I saw a lot of people who were unable to speak, had broken legs."
Another passenger, Brett Byatt, told the BBC that only "three to four of us were uninjured in a full carriage".
"Everyone else had either a serious wound that was bleeding profusely, or a situation where they couldn't stand, or couldn't move their neck, or I saw a woman's snapped leg," he said.
In recent years, Britain's railways have had one of the world's best safety records.
One passenger was killed in a collision between two trains in Wales in October 2024.
That was Britain's first fatal crash involving multiple trains for more than a quarter of a century.
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