The Association of American Railroads, the trade group that coordinated the three-day suspension by North America's 11 major freight carriers, was concerned terrorists could target trains to retaliate for the bombings.

"We had to assess the potential problems and whether there was any credible threat to the railroads as a result of the bombings of Afghanistan," said John Bromley, spokesman for Union Pacific Corp. "This was a special event, and we wanted to make sure we were equipped to handle these special commodities."

The industry stopped transporting 46 types of "dangerous military goods and chemicals," when the attacks began Sunday, according to the National Industrial Transportation League, a trade group.

League spokesman Ed Rastatter said banned cargo included chlorine, certain pesticides, and fuming sulfuric acid.

Tom White, a spokesman for the railroad association, said many of the materials are essential to the economy.

"They have to move, and the railroad is the best way to move them," White said. "But we had to make sure we had the proper security measures in place first."

Industries dependent on shipping the chemicals welcomed the lifting of the ban Wednesday. Bill Ramirez, manager of Ashland Chemical Inc., in Birmingham, Ala., said a prolonged ban could have been costly.

"One rail car is about 3 1/2 trucks," Ramirez said.

During the self-imposed "red alert," rail officials worked with the U.S. Department of Transportation and federal intelligence agencies to make sure the lines were safe.

Other measures introduced since Sunday include:

_Increased security at bridges and tunnels and more track inspections.

_Limiting the release of information _ much of which had been available on the Internet _ about what trains are carrying and their routes.

_Preventing cars carrying hazardous materials from passing major public events.

_Activating a 24-hour command center that coordinates the locations of freight trains and can be immediately linked to federal security agencies and railroad operations centers.

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On the Net:

National Industrial Transportation League: http://www.nitl.org

Association of American Railroads: http://www.aar.org

Union Pacific Corp.: http://www.up.com