BEVERLY - It's 600 feet long, weighs 200 tons, holds 20,000 gallons of water and spews enough sparks to light the occasional backyard on fire.

And it's been traveling through the North Shore under the cover of darkness for the last four nights.

The beast in question is called a rail-grinding machine. Since Sunday, it's been riding the railroad tracks from Beverly to Gloucester and Newburyport, scraping the steel rails with its 88 grinding stones as part of a maintenance program to keep the tracks in good shape.

For all of its size and fury, the 11-car train-like machine might have gone unnoticed by most people because it runs mainly at night. But the machine has also been blowing its horn as it travels the tracks, catching the attention of at least some slumbering residents.

"It was loud enough to wake my wife and me up out of a sound sleep," Robert Torkildsen of Beverly said.

Susan Otis of Hamilton said the horn woke her about 3:30 a.m. on Monday or Tuesday.

"We thought, 'Oh my God, somebody must be on the tracks with a car,'" she said. "We thought we'd hear a crash."

Despite the fact Beverly has 18 railroad crossings, among the most in the state, the sound of train horns is actually rare. Beverly and Hamilton are both "quiet zones," meaning trains can sound their horns only in case of emergency.

Lannie Deserly, the field clerk on the grinding machine, said he wasn't aware of the local rules regarding horns. His company is based in Minnesota and is hired by the Massachusetts Bay Transportation Authority to grind the tracks.

"I suppose we could get away without sounding the horn," Deserly said. "We have a representative from the MBTA with us, and he hasn't said anything to us."

The MBTA did not return phone calls seeking comment.

The grinding machine has been sitting on the tracks near Beverly Depot during the day for the last four days. It was scheduled to finish its work on the North Shore line by this morning and be headed for Boston, Deserly said.

The machine is used to reshape rails flattened out by use, said Johnny O'Brien, who heads the night maintenance crew on the machine.

"It improves traction and fuel efficiency," he said. "Basically, it smooths out the ride."

The grinding machine travels throughout the country and into Canada. The crew is on the road for six weeks at a time, then home for two weeks.

O'Brien, 53, lives in Missouri with his wife and 7-year-old son. As he stood beside the machine yesterday, his face was covered with dust.

"It's a dirty job," he said. "We've had people who have quit the day they started. All of a sudden they'll disappear."

Eighty-eight computer-controlled grinding stones create huge sparks as the machine travels along the rails at night. The sparks can start fires, so the machine is equipped with a 20,000-gallon water tank and hoses.

O'Brien said it's not unusual for passers-by to wonder if the machine is on fire.

"It's quite the light show," he said. "It's very pretty."