Rush-hour commuters were treated to an unusual display of aerial activity yesterday at Penn Station, where travelers reported seeing two roaring Black Hawk helicopters follow a high-speed Acela Express train.

The aerial convoy was sighted about 4:30 p.m. and was part of a military exercise, said Amtrak spokesman Dan Stessel. He would not provide details about it but said Amtrak periodically conducts drills with law enforcement agencies.

Maryland National Guard spokesman Maj. Charles Kohler, whose office is near Penn Station, said the helicopters were UH-60 Black Hawk aircraft. The front-line utility helicopter is used by the Army and National Guard to deploy troops and heavy equipment.

It could not be determined yesterday what agency or military branch conducted the exercise.