The death of a teenager and the maiming of another at a transport interchange is potentially an escalation of Sydney's suburban "postcode wars".
The 19-year-olds were stabbed after a quartet of masked people ambushed the pair at a bus interchange opposite Mount Druitt train station late on Wednesday,
A third man, believed to be waiting for a bus and unrelated to the groups, was also assaulted, police said.
Paramedics treated the teens at the scene but one of them, who was stabbed in the chest, could not be revived.
The other 19-year-old was stabbed through the arm and was under sedation in hospital on Thursday morning.
"That male is extremely lucky to survive," Superintendent Darrin Batchelor told reporters.
"He sustained a stab wound ... that went through the entirety of his arm and then penetrated his chest."
Police on Thursday morning were scouring the scene, which extends through the bus interchange, across a busy road and over the railway tracks, where the offenders were seen fleeing.
The railway, linking Sydney to the Blue Mountains and beyond, was closed for some time as a police emu line closely examined any evidence left on the tracks.
Links to the so-called "postcode wars" and a gang from Doonside were being investigated.
"Our initial reports from the scene was there was some conversation about 'Are you 67?' - one of the street gangs that are out here in western Sydney," Supt Batchelor said.
"The attack (was) callous, vicious and without regard to the victims themselves."
The four offenders behind the "violent, frenzied" attack were all dressed in black and wearing balaclavas, police said.
