One of the victims was struck over the head with the weapon by Walker, the court heard, as he attempted to attack a second man on board the train.
The teenager exited the train at Seven Sisters, where he attacked another man on the platform before running back into the carriage to chase his previous two victims.
Terrified passengers fled the carriage as Walker threw away the hammer and a screwdriver on the platform on his way out of the station.
He was arrested at his home a week later after forensic cops identified his DNA on a carton of apple juice he left at the scene along with the discarded weapons.
In his police interview, Walker refused to give any explanation as to why he randomly attacked the men other than saying he was “stressed” during the day.
His first victim suffered lacerations and a fractured skull, spending ten days in hospital, while the third victim also received treatment in hospital for a head wound.
The second victim suffered minor injuries which did not require hospital treatment.
Detective Inspector Gary Pinner said: “This was one of the most shocking and senseless acts of violence I have come across in my entire career, and it was inflicted on three innocent men without any provocation.
“Walker is clearly an incredibly dangerous individual, and it is only through luck that his victims weren’t more seriously injured. While this was a truly horrifying experience for them, I’d like to reassure the public that incidents like this are thankfully extremely rare on the railway network.
“We are also lucky to police a CCTV rich environment which captured the incident in its entirety and proved invaluable in helping to bring Walker to justice. I am relieved to know he’ll now be spending the foreseeable future behind bars.”
Walker was sentenced at Southwark Crown Court on June 22 to 12 years behind bars, with an extended licence period of five years.
He had previously pleaded guilty to grievous bodily harm (GBH) with intent, GBH without intent, attempted GBH with intent and possession of an offensive weapon at the same court in May.