Two terrifying rail security breaches occurred within hours of each other in New York City as authorities remained on alert for an al Qaeda attack, the New York Post reported Monday.

In one incident Sunday, near the World Trade Center, a man slipped into a train tunnel and walked all the way to from Manhattan to New Jersey before saying that he left a bomb in the tunnel.

That scare — and an unrelated escapade involving four “urban explorers” infiltrating an under-construction subway tunnel — came just days after the FBI warned that al Qaeda could be targeting US trains.

“How this could happen is unbelievable,” one law-enforcement source said. “Everybody’s warning us about tracks and subways and where they’re going to attack.”

There was no bomb on the tracks, police added.

Officials said that 20-year-old Reymundo Rodriguez, of Bayonne, N.J., hopped down onto the tracks in a Manhattan tunnel from a station being patrolled by two Port Authority cops. Rodriguez then walked the two miles to Jersey City.

A Port Authority contractor, Lee Anderson, spotted him exiting at around 3:00am local time Sunday and called police.

“I just put a bomb down on the tracks,” Rodriguez allegedly told officers.

The tunnel was shut down while the Joint Terrorism Task Force and bomb-sniffing dogs searched for a device.

Anderson told cops, “I asked him what he was doing in the tracks, and he said, ‘The train never came … so I decided to walk.'”

Rodriguez was charged with criminal trespass, evaluated at a hospital and released.

Separately, at around 4:30am local time Sunday, cops arrested four men who allegedly sneaked into the Second Avenue subway tunnel carrying Roman candles and cameras. The men told cops they were part of an “urban explorers” group and that they planned to use the fireworks for light for photos.

Eric Ruggiero, 25, of Manhattan; Jacob Bloom, 21, of Glen Cove, Long Island; Braiden O’Sullivan, 21, of Connecticut; and William West, 27, of Massachusetts, were charged with criminal trespass after Harlem resident Jerry Jackson alerted cops that the four descended into the tunnel around 112th Street.

READ MORE: NY POST