The 17-year-old was discovered with stab injuries outside Leyton station following the broad daylight horror.
He has been airlifted to hospital in a life-threatening condition.
There have been no arrests in connection with the stabbing, which took place at around 1pm.
Trains are still running through the station but are not currently stopping at Leyton and roads have been closed off while police investigate.
A Metropolitan Police spokesperson said: "Police at scene of stabbing outside #Leyton station #WalthamForest – 17yo male found with stab injuries.
"Anyone with info call 101 ref 3114/13Mar".
It comes as Britain battles a knife crisis with 42 people stabbed to death across the country so far this year – including 12 in just two weeks.
On Monday night, a 19-year-old boy was seriously injured after being stabbed in the chest outside a DLR station in Lewisham, South East London.
Jodie Chesney and Yousef Makki, both 17, were stabbed to death in a devastating weekend of violence earlier this month.
Meanwhile, Scotland Yard chiefs have unveiled plans to tackle the knife epidemic by stop-and-searching 1,000 suspects every day.
It comes as the UK murder rate soars to the highest number for a decade.
Knife crime has also increased by eight per cent as violence spirals out of control across the country.
Philip Hammond today announced a £100 million funding package to tackle Britain's knife crime epidemic.
The cash will go towards violent crime units in the seven worst affected UK cities.
It is a major victory for Home Secretary Sajid Javid, who demanded the cash after the surge in knife killings this year.
The money will also help pay for police officers overtime as they struggle to combat spiralling knife crime.
More to follow…
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