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The Coroner's Office has confirmed it is treating the tragedy at Frankton Train Station on Tuesday morning as a suspected suicide.
The woman was 27 years old and the baby was less than a week old, the coroner confirmed.
Coroner Bruce Hesketh is now investigating the cause and circumstances of the death.
Police told the Herald they would not be releasing any further information.
Emergency services were called just after 8am and found the mother and baby were dead.
KiwiRail acting group chief executive David Gordon said the train driver has been put on leave and was being offered counselling and any other support.
"These types of events are deeply traumatic for those involved, and our thoughts are with all those affected," Gordon said.
Shortly after the pair died, several people gathered at the scene and could be seen hugging and comforting each other.
The area was cordoned off while police investigated.
Mothers Matters communications professional Claire Henry said it was highly concerning how prevalent mental health issues were among women who were also finding it harder and harder to get support.
Motherhood was becoming more stressful and the midwife shortage and limited choices available to birthing women was increasingly concerning and could be detrimental to a mother's wellbeing, she said.
Mothers Matter is lobbying the Government to establish a fund that enables women to use the money to support their personal needs such as mental health or physio for pelvic floor issues.
They also wanted women to have better access to birthing facilities of their choice and to have better mental health screening from pregnancy until way after birth.
Henry said the Hamilton train incident was extremely sad and so many stories around women's experiences were going unheard.
"There must have been some red flags there."
Where to get help:
• Lifeline: 0800 543 354 (available 24/7)
• Suicide Crisis Helpline: 0508 828 865 (0508 TAUTOKO) (available 24/7)
• Youthline: 0800 376 633 or text 234 (available 24/7)
• Kidsline: 0800 543 754 (available 24/7)
• Whatsup: 0800 942 8787 (12pm to 11pm)
• Depression helpline: 0800 111 757 or text 4202 (available 24/7)
• Anxiety helpline: 0800 269 4389 (0800 ANXIETY) (available 24/7)
• Rainbow Youth: (09) 376 4155
If it is an emergency and you feel like you or someone else is at risk, call 111.