CCTV footage is being examined to see if it shows a mother killing herself and her two children by jumping from a train station platform.
Forensic teams examined the scene
They were all hit by a Heathrow Express train travelling at up to 100mph in Southall, west London, on Wednesday.
The married 27-year-old, from Southall and her five-year-old daughter died instantly. Her baby boy died later at Ealing Hospital from multiple injuries.
Police say no-one else is being sought over the deaths.
There has been confusion over the baby's age with police saying he was nine months old and hospital officials believing he was aged 23 months.
'Driver in shock'
Southall station was shut from about 1320 BST, when the three were killed.
It reopened just before 2000 BST after forensic examinations had been carried out and the track cleared.
About 80 passengers on the high speed train, which was heading towards Heathrow airport, were kept on the train for three hours and then let off at the next stop at Hayes.
The driver had to be treated for shock.