Anxious and struggling under a mistaken belief that she had harmed her baby girl, Melissa Arbuckle believed she had run out of options.
The new mum was pre-occupied by concerns she had caused shaken baby syndrome in three-month-old Lily by rocking her bassinet too strongly.
The 32-year-old first-time mum told medical teams she was struggling but believed voices telling her she was a bad mother and that Lily would never love her.
She sent a text message to her husband telling him they were going for a walk because Lily wouldn't settle.
Arbuckle waved to a train and then laid on the tracks near the family's Upwey home in Melbourne's southeast just after 5pm on July 11 last year.
The approaching driver used the emergency brakes and sounded his whistle but struck the mother and baby. Lily suffered significant and ultimately fatal head injuries.
Arbuckle was also injured and tried to further self-harm as people rushed to help.
She pleaded guilty to infanticide, a rare charge that applies only to mothers who kill children under the age of two while suffering a "disturbed" mind linked to birth.
Arbuckle's decisions that day were made in the midst of a spiralling depression, Supreme Court Justice Jane Dixon said on Thursday.
She was suffering postpartum depression with psychotic features and postpartum psychosis.
While Arbuckle believed she deserved to suffer for her crime, Justice Dixon spared her a prison sentence and instead ordered her to continue treatment and supervision for three years.
Lily's birth - via emergency C-section - was traumatic for Arbuckle, who also struggled with breastfeeding and settling her baby, the judge said.
Arbuckle believed she was broken and her daughter could see that.
She told doctors and maternal health specialists she was struggling, not coping and felt like she was missing something.
"You became increasingly certain Lily had become different and had lost her spark," the judge told Arbuckle.
Justice Dixon said Arbuckle was prepared to consider that she had a mental illness but primarily believed she was bad and should die.
Lily's father says his daughter deserved more.
"I loved Lily more than life itself. Seeing Lily at the hospital broke my heart. My dear little girl was gone and I just felt so helpless," he said in a statement read to the court.
Experts said Arbuckle's pregnancy was wanted and planned.
Being a high achiever whose self esteem and self worth were based on achievement, Arbuckle was predisposed to mental illness because of her perfectionist personality.
Giving birth destabilised the balance of her mind, resulting in psychotic illness.
"Prior to the incident her anxiety increased, her sleep deteriorated and she became focused on a fixed, delusional belief that she had harmed her child and they were both broken," Justice Dixon said.
Arbuckle is in full remission from her psychosis but has symptoms of post traumatic stress disorder. She is remorseful and struggles with profound guilt and sadness.
The court heard if Arbuckle was to have children in future her chances of postpartum psychosis are 50 per cent, but Justice Dixon said her current position was she would not have children.
Justice Dixon said Arbuckle was fully engaged with psychiatric support and had done everything possible to recover.
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