The 52-year-old victim, a conductor who was off duty and not in uniform, was on the J train at around 11:20 p.m. near Fulton and Crescent streets.
A man passing through train cars slashed Gerard Sykes with an orange box cutter.
Sykes was slashed from behind his ear to his left eye.
He was taken to Jamaica Medical Center where he underwent two emergency surgeries.
The father of three was last said to be unconscious and in critical condition.
The suspect ran away from the train at the Crescent Street station. No arrests have been made.
The incident was one in a string of violent encounters on the subway Wednesday.
On Thursday, at least two more incidents were reported: a 2 train operator in the Bronx was punched and knocked unconscious and a rider in a wheelchair somehow ended up below a subway train at the Barclays Avenue station.
The incident in the Bronx happened around 1:30 p.m. between 238th Street and Nereid Avenue. The operator smelled cigarette smoke and opened the cab door to investigate when the suspect kicked the door back and proceeded to strike the 39-year-old in the head.
Sykes' aunt spoke to the Transport Workers' union outside of the hospital on Thursday.
"People are scared, they are scared and now it's happening to your workers, I know I would not ride the subway, I would not ride the bus because you don't know what is going to happen," Cassandra Sykes said.
Union and MTA leadership joined forces to slam Mayor Bill de Blasio and renew their calls for more police.
"Our ask is from the city additional police officers riding on the subway platforms, riding on the trains to protect riders, customers and our employees, like Mr. Sykes," MTA Chairman Patrick Foye said.
Crime in the subways, particularly assaults, is up, despite plunging ridership -- from over five million a day to under one million in April of last year.
In the months that followed, riders have been attacked and transit workers have been harassed and assaulted.
"Our members are getting spit on, they're getting punched, they're getting stabbed, they're getting murdered and then you have a mayor that says there's no problem," said TWU Local 100 president Tony Utano.
In February, an additional 600 officers were surged into the system after a series of shoving incidents and a stabbing spree by a homeless man.
When Mayor Bill de Blasio was asked about safety in the subway Thursday, he said the NYPD is doing a great job and overall crime in the subways has gone down.
"The NYPD has made clear that they'll make adjustments whenever needed, the officers that were surged into the subway helped address some real issues and whenever the have to do that again they will," de Blasio said.
The head of NYC Transit says that makes it seem like the extra officers have already been pulled back at a time when the MTA is trying to reassure passengers it's safe to ride again.
"We're talking about the next three to six months, give us a buffer so that we can get ridership back, get us to that tipping point where there's more safety in numbers," Sarah Feinberg said.
ALSO READ | Exclusive: NYC Transit Authority president rides subway amid spike in violent crime
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