A hardworking father of two was killed Sunday when a stranger shoved him off a Bronx subway platform and into the path of a train — as his horrified wife looked on, family and police sources said.

A suspect was spotted minutes later boarding a city bus — and then hopping off to coolly smoke a cigarette.

Victim Wai Kuen Kwok, 61 — who worked in a kitchenware factory in Brooklyn so that his two kids wouldn’t have to struggle — was on his way to Chinatown with his wife to do their weekly grocery shopping, the couple’s grieving son told The Post.

Kwok and his wife, Yow Ho Lee, were standing on the platform at the East 167th Street station in Highbridge shortly before 9 a.m. when the killer suddenly pushed the Chinese immigrant in front of an oncoming D train.

“He doesn’t say a word to them,” a source said. “He just shoves the guy and takes off.”

The motorman witnessed the push and slammed on his brakes, but it was too late, a transit source said. The lead subway car struck Kwok while his body was still in midair. Several of the train’s cars then rolled over him.

Police released surveillance video of a middle-aged, balding man dressed in a black leather jacket, black pants and white sneakers calmly sauntering outside the station after the crime. He then hopped onto a Bx35 bus at Grand Concourse and East 167th Street, where at least one straphanger also was waiting because train service was halted after Kwok’s death, sources said.

The straphanger witnessed the fatal push — and soon came to believe that he and Kwok’s killer were on the same bus, sources said.

The “customer recognized him and thought he was the subway pusher,” a source said. “The person was too scared [to speak up] and feared for his life, so he waited until the end of the line [before alerting the bus driver].”

By that time, up to 30 minutes had passed since the man he supected was the subway pusher had gotten off, sources said. The bus driver quickly called police, who tracked down surveillance video near where the suspect had exited the bus just a few blocks from the subway station.

Footage showed the man strolling into a bodega and then coming out again to smoke a cigarette. Police were dusting the bus for fingerprints Sunday night.

Kwok’s son, choking back tears at the family’s Findley Avenue apartment, called his father “a good person” who worked six days a week at a factory in Dumbo that produces kitchen accessories.

“He provided for his family,” said the son, one of Kwok’s two boys, adding that the Hong Kong native dreamed of giving them a better life. “He wanted us to be successful. His plan was to work and get his sons through school,” the young man said.

Kwok’s shocked wife, 59, was taken to Bronx Lebanon Hospital.

Additional reporting by Daniel Prendergast, Dana Sauchelli and Priscilla DeGregory