A mother and her two children have died after she jumped in front of a Heathrow Express train travelling at speeds of up to 100mph.
CCTV footage is being examined
The married 27-year-old, from Southall in west London, jumped with her five-year-old daughter and baby son from a Southall platform on Wednesday.
Mother and daughter were killed instantly, while the baby died later at Ealing Hospital from multiple injuries.
Police said they were not looking for anyone else over the deaths.
The age of the baby boy is not clear - police said he was nine months old, but hospital officials believe he was aged 23 months.
Driver in shock
CCTV images are being examined to see if the incident was caught on camera.
British Transport Police chief inspector Willie Baker said the three were hit by a train travelling towards Heathrow at 1320 BST.
He said the train came to a halt about a quarter of a mile down the track and that about 80 people were on board.
The passengers were kept on board the train for three hours while forensic tests were made and witness statements taken.
The train was then taken to Hayes station where they were allowed off and further detailed forensic tests carried out.
The track was cleared by about 1745 BST and Southall station reopened at 1950 BST.
The driver was later treated for shock.