A teenage boy who admitted stabbing a waitress at a Perth train station during a violent robbery and leaving her unconscious in a pool of her own blood started using methamphetamine at the age of 14 and had a significant cognitive impairment, a court has been told.

The Perth Children's Court also heard the 15-year-old, who cannot be identified, was involved in a serious of violent incidents including the bashing of a taxi driver in the months leading up to the latest attack.

He was found in possession of weapons including a tomahawk, metal hatchet, meat cleaver, a 30-centimetre kitchen knife he kept tucked into his waistband and a hammer, which he used to threaten a passer-by he tried to rob in Northbridge.

The waitress, Italian national Stella Trevisani, was walking through Claisebrook Cove to Claisebrook Train Station in East Perth on the evening of March 17 when she noticed three men.

They followed her for about a minute before the boy stepped forward and demanded the phone.

She resisted but he told her: "Give me the f***ing phone or I will stab you all over", the court heard.

She dropped the phone but he continued to stab the young woman, leaving her with five deep lacerations on her arms and leg, one of which required a vein graft due to its severity.

Woman considering tattoos to cover scars

Ms Trevisani spent a significant period in hospital and will need ongoing rehabilitation.

The court was told she had ongoing trauma from the incident, with large scars on both arms and was considering tattoos to cover them.

"She is re-traumatised every time someone stares at her," prosecution lawyer for the state, Sean Stocks, said.

"She spent months not able to [go to] the toilet herself."

Judge Julie Wager read a letter from Ms Trevisani detailing how she thought she would die from her wounds, and she could not work for three months.

"I have scars not only on my body, deeply in my soul as well," the letter said.

The attack also hit her extended family hard, with her grandmother left distressed and her brother, who is in Italy, requiring counselling over his fears for his sister.

When her grandmother saw the news on television in Italy, she fainted.

The court heard when the boy was arrested 100 metres away, covered in blood and in possession of the knife and phone, he was behaving in an "agitated way", spitting on officers and was clearly intoxicated.

Police at the Claisebrook train station where Ms Trevisani was stabbed. ( ABC News )

Charges over mounting violence in lead-up to station attack

His actions also constituted a breach of an earlier intensive supervision order relating to other violent acts, the court heard.

In August, he was charged by police after being found carrying a metal hatchet and knife while hanging around the Commonwealth Bank in Murray Street.

In December, he was charged after using a meat cleaver to smash the windows of a car at Mandurah Forum shopping centre.

At a later incident at the same centre, when security staff in Kmart pulled him aside over suspicion of stealing, he allegedly pulled out a tomahawk and said "there you go".

In February, he threatened a Vietnamese man who spoke little English on the street in Northbridge, demanding money.

When the man denied he had money, the boy allegedly chased him into a deli and threatened him and staff with a hammer.

Earlier in the day before he attacked Ms Trevisani, he stole two two-litre bottles of spirits from a liquor store.

The court ordered the boy be subject to a life violence restraining order, prohibiting him from coming into contact with Ms Trevisani.

Cognitive impairment impacts understanding: lawyer

The boy's defence lawyer, Christian Miocevich, conceded the offences — to which the boy has pleaded guilty — were extremely serious.

He said the boy, who is now 16, had struggled with life in custody and had made attempts at self harm.

The boy felt bad about the attack, and had nightmares about it, his lawyer said, although he could not recall much of what happened at the train station.

But he said the boy "presents as someone with problems and needs".

Prior to the attack he was going from house to house and living on the streets.

"He was feeling let down by his parents," Mr Miocevich said.

The boy's father was in court today, and Mr Miocevich said while he had his own difficulties, the man planned to "get his own life together" and have the boy come into his care once he had served his sentence.

Mr Miocevich said the boy had significant treatment needs and one medical expert found he was suffering from foetal alcohol spectrum disorder (FASD) and cognitive impairment.

"He is unfortunately a product of his environment," the lawyer said.

Police shut down access to parts of the Claisebrook Train Station in East Perth after the incident. ( ABC News: James Carmody )

Mr Stocks said while the FASD diagnosis may be a part of his condition, there were other factors including the boy's own drug use and his mother's situation while pregnant, as she was using methamphetamine and was assaulted during the pregnancy.

The judge accepted the state's submission post-traumatic stress disorder (PTSD), attention deficit hyperactivity disorder (ADHD) and attachment disorder were all tied up with the boy's neurological and intellectual issues.

But Mr Stocks argued the boy's actions had resulted in people losing faith in being able to safely use public transport.

"Anybody, particularly women, are entitled to feel safe walking the streets of Perth," he said.

"This type of offence causes people to be fearful of using that system (public transport).

"This poor woman was simply unfortunate. She was simply in the wrong place at the wrong time."

He said if not for members of the public coming to her aid, she could have died.

"He stabbed her, he inflicted this injury on her and he walked away and left her behind," Mr Stocks said.

"It was exceptionally callous to do that."

Boy not a vehicle for deterrence: Judge

In her sentencing remarks, the judge said if he had been an adult, he would have been liable to receive life imprisonment for his crimes.

Judge Wager said reports indicated he had started using cannabis at 11 and methamphetamine at 14.

He was taken into care after being neglected and being exposed to violence by his parents, who were substance abusers themselves.

She said because of these factors he was not a good "vehicle for deterrence" but the crimes were too serious for anything other than a period of detention.

The judge also noted psychological testing showed he was at a high risk of re-offending.

She sentenced him to four years in detention, but backdated it to March 17 when he was taken into custody.

He will be eligible for supervised release in March 2020.