Amtrak's Night Owl train averaged 91.8 mph for the last mile and a quarter into Boston's Back Bay Station Wednesday morning before derailing on a curve and slamming into a crowded commuter train, the National Transportation Safety Board said yesterday.

At least 264 people were injured. Among them, speaking separately from two different hospitals, the apprentice engineer who was at the throttle and the experienced engineer who was monitoring his performance told investigators essentially the same story: that the brakes failed to work.

The apprentice, Richard Abramson, 41, said he attempted to set the air brakes three times as they roared toward the tunnel into the underground station, but they failed to "set up."

At the last minute, the experienced engineer, Willis Copeland, used a separate emergency brake valve, according to Abramson. A train, however, does not stop quickly, especially from such high speeds. The speed limit on the curve going into the station is 30 mph and some engineers estimated that 60 mph would cause a derailment.

"He {Abramson} made three attempts to slow the train, and in each instance, in his opinion, the brakes failed to set up and slow the train," said Susan M. Coughlin, vice chairman of the safety board.

Copeland, while having greater injuries than Abramson and unable to talk as extensively, told a story with "no glaring differences," Coughlin said.

Officials in Boston said 14 of the injured were hospitalized. Last night, only the two Amtrak engineers and four other people remained in intensive care.

The Night Owl, completing an overnight run from Washington, was entering the station Wednesday with about 190 passengers when it derailed and its two locomotives slammed into the rear of a stopped commuter train with 900 passengers on an adjacent track.

Coughlin said it is clear that the train derailed on a 30 mph curve, but she stopped short of giving speed as the cause of the wreck.

"At the point he {Abramson} entered the curve, he made the statement that the train went straight," Coughlin said.

Tests on the cars on the Amtrak train that were not destroyed showed their brakes were in working order, she said. More extensive tests of the rest of the braking system will be necessary to determine if there was a defect or if the brakes actually went into emergency mode, she said.

The injury toll was minimized, authorities said, because the commuter train was bidirectional, with the locomotive at the rear and the engineer in a cab at the front. The locomotives absorbed much of the impact, rising off the ground and almost penetrating the street above.

"If it had been the other way around, you would have probably had 150 dead," a federal investigator said.

Trains entering Back Bay move for many miles on track rated at a 100 mph maximum. About a mile entering the south portal of the station tunnel, engineers should begin braking for the 30 mph curve about one-eighth of a mile inside the tunnel and just before the passenger platforms.

A train traveling somewhat faster than 30 mph is not likely to derail on such a curve, authorities said, although its passengers would have a rough ride. At about 60 mph, according to engineers' preliminary calculations, it would jump the track.

Amtrak has several sophisticated devices to check that engineers do not speed through restrictive signals or fall asleep at the throttle.

Each locomotive has an "alerter" designed to awaken sleeping engineers or stop a train in the case of debilitating illness. Every 20 seconds, an engineer must touch, then pull back from any metal surface on his control console. If he does not touch the console or touches continuously for more than 20 seconds, a light flashes, a horn sounds and, 15 seconds later, the brakes are applied automatically.

That same light-horn-brakes scenario begins if an engineer runs through a restrictive trackside signal light too fast without applying brakes to slow down.

However, if a signal is clear, an engineer can pass it at any speed without tripping the alerter, no matter what the track speed restriction. An Amtrak spokesman said the last signal passed by the Night Owl was within a half-mile of the tunnel portal and clear.

The primary brake on trains is an system in which air is continuously pumped into a brake line at high pressure running the length of the train. To apply brakes, air is released from the line, allowing compressed air from storage tanks on each car to flow into cylinders that apply pressure to brake shoes. When the shoes press against the wheels or against disks on the axle, the brakes are said to have "set up."

The average speed was determined by records from Amtrak's South Boston Dispatch Center, which showed that the train passed a lineside track signal at Ruggles Road at 8:21 a.m. and 49 seconds, and derailed on the curve in the station 6,590 feet later at 8:22 and 38 seconds.

In a quirk that Coughlin called "regrettable," the locomotive speed recorder on the lead diesel unit was inoperative, and the recorder on the second unit quit just five miles from the accident scene. Coughlin said the board does nor think the recorder was tampered with. "It just stopped," she said.

"We will never know the speed," she said. "It's regrettable about the loss of data."

Special correspondent Christopher B. Daly in Boston contributed to this report.