Plainclothes officers were shifted into uniform and more cops than usual watched platforms and stations, especially at hubs such as Penn Station, Grand Central and Bowling Green.

Pairs of officers walked up and down platforms at Times Square and guarded the entrance to the Port Authority Bus Terminal.

And police from the Metropolitan Transportation Authority also flooded Metro-North and Long Island Rail Road lines - supplemented by state troopers ordered up by Gov. Pataki.

"We're on trains, we're in stations," said one cop. "We're all over the place today."

Investigators also left for Spain to monitor the probe and hopefully strengthen their counterterrorism strategy.

Yet the bombing of the commuter trains in Madrid shows how vulnerable railways can be to terror attacks.

Experts say security on U.S. commuter trains has improved in the 30 months since the Sept. 11, 2001, attacks, with increased awareness about unattended bags and improved coordination between agencies.

The sheer number of riders makes it all but impossible to screen passengers like airlines do, and the miles of rails and tunnels limit efforts to guard all spots.

"It could happen here," said Eva Lerner-Lam of the Palisades Consulting Group in Tenafly, N.J. "The transportation systems are open."

Lerner-Lam credited transit officials with taking concrete steps to improve security, such as installing more surveillance cameras and eliminating nooks and crannies where terrorists could hide in stations and tunnels.

But Sen. Chuck Schumer (D-N.Y.) said officials have to do much, much more.

"The bottom line is, we are vulnerable," Schumer said.

He called the 11/2-mile tunnel leading out of Penn Station a "death trap," lacking emergency ventilation shafts and proper evacuation routes.

Amtrak requires passengers to show photo identification when buying tickets and receives daily threat assessments. National Guard troops watch over Penn Station and other key stations.

But with 65,000 passengers on 260 trains, Amtrak officials admit they can't be everywhere at once and still get the trains to run on time.

"The best way to prevent this is by intercepting the perpetrators before they do it," said Amtrak spokesman Clifford Black.

The Port Authority is already on a heightened state of alert, and no additional security was placed at area airports or on PATH trains, which carry about 180,000 people a day, including 56,000 who use the World Trade Center and 33rd St. stations, said PA spokesman Steve Coleman.

NJTransit also carries 58,000 passengers to Penn Station each day - illustrating how difficult it is to safeguard every train that comes into the city.

Mayor Bloomberg called the attacks in Spain a wakeup call to the carnage that terrorists could wreak on New York trains that carry millions of passengers a day.

"It goes to show we still live in a very dangerous world," Bloomberg said. "We cannot afford to let our guard down."

For some, the extra police patrols yesterday were comforting.

"They can't prevent it 100%, but they can deter it," said subway rider Jason Latty, 25, of St. Albans, Queens. "They make me feel better."

But others questioned whether the extra measures would do any good.

"New York City is so big, if someone wants to do it, they'll do it," said Larry Davis, 49, of Jamaica, Queens.