It's a bird! It's a train! No, it's . . . an off-duty firefighter!

Superhero fireman Adam Rivera, 30, leaped two subway tracks in a single bound and skipped over a pair of third rails to rescue a man who fell off the platform.

Rivera risked life and limb to save the life of Marco Delamo, 45, who passed out, landing on a downtown N/R/Q/W train track at the Union Square station Friday at about 10 p.m.

Delamo suffered a fractured skull but is improving.  "He was like a Superman," Delamo said of his savior in a bedside interview at St. Vincent's Hospital.

Rivera's late-night heroics began as he stood with his girlfriend near the northern end of the uptown platform. The couple was headed home to the Upper West Side after spending the evening at an Indian restaurant celebrating their seventh anniversary.

As they waited for the train, Rivera looked across the tracks and saw a commotion.  "I stepped closer to the edge and saw a man lying on his back on the tracks," Rivera told The Post. "People were panicking, but nobody was doing anything."

That's when Rivera sprang into action.

The Engine 10 firefighter saw an uptown train entering the station, so without uttering a word to his stunned girlfriend, he hopped onto the uptown tracks. Then he bounded over the third rails and squeezed between the pillars separating the two sets of tracks to get to the sick man.

"I thought to myself, 'This is my job -- I'm a New York City firefighter, and I have to do something,' " Rivera said. "I knew I had one chance because the train was coming, and there was no time to be afraid."

When Rivera reached Delamo, he was unconscious, but his legs, which were sprawled across the tracks, were moving.