The subway system's top cop yesterday said there is an around-the-clock effort to find the man suspected of shoving a 61-year-old man into the path of a D train, killing him.

NYPD Transit Chief Joseph Fox opened testimony to MTA officials at a board meeting yesterday with condolences for Wai Kuen Kwok's wife, Ho Yow Lee Kwok, 59, and his family.

"I want all to know that our [Chief of Detectives Robert Boyce] and our transit members have been working constantly throughout the night and this will be a 24/7 operation until we identify who did this and bring him to custody," Fox said.

Kwok's grisly death came at the hands of an apparent stranger shortly before 9 a.m. Sunday at the 167th Street station at Grand Concourse in the Bronx, according to the NYPD.

The suspect has not been identified, an NYPD spokesman said Monday evening. But video surveillance was released Sunday evening that shows the suspect on the sidewalk near the station and exiting a Bx35 bus he took to 170th Street and Jesup Avenue, according to the spokesman.

Meanwhile, the MTA's transit chief said an engineering study is under way to look at platform doors, which have been touted as a way to prevent death and injury from accidental falls, suicide attempts and homicides. A consultant contract to an engineering firm STV was awarded last month, according to NYC Transit President Carmen Bianco.

Bianco said installing doors raises several issues that need studying, such as signal technology, station and platform layout, subway system age, and passenger flow.

"There's a whole host of items that have to be looked at from an engineering perspective that will allow us to understand the possibility of going forward with the project," he said.