The 33-car CSX freight train blasted its whistle as it thundered toward a railroad crossing in Conasauga, Tenn.

"Hey! Hey!" the engineer shouted when he saw a school bus rumble toward the tracks, then slammed on the emergency brakes.

It was too late.

The 2,465-ton train smashed into the side of the bus. One witness said the crash sounded like "damn thunder or a bomb blown up."

Two girls, 7 and 9, and a boy, 9, were killed in that crash in 2000. Three more children were seriously hurt, one of them the bus driver's daughter.

The National Transportation Safety Board investigated the crash and in 2002 recommended states make it a high priority to improve safety at ungated railroad crossings -- those without barrier arms that lower to block traffic -- used by school buses.

But since then, only 12 states have made satisfactory progress, the NTSB says. And seven of the 10 states with the most collisions between trains and all kinds of vehicles at grade crossings have largely ignored the recommendation. They are Alabama, California, Georgia, Illinois, Louisiana, Mississippi and Ohio.

"Children continue to be unnecessarily killed in school bus accidents at grade crossings," NTSB Chairman Ellen Engleman Conners said in a recent interview. "Children's lives can and will be saved if state authorities adopt the simple safety measures that the board recommended. With the school year beginning, action is needed."

Vehicles and trains collide an average of nine times a day. More than 1,000 people have been killed in the four years since the Tennessee crash. In the first five months of this year, there were 1,205 crashes, including four involving school buses, and 155 deaths.

The 82,000 crossings where there are no gates present the greatest danger -- the accident rate is seven times that for crossings with gates that block vehicles.

A key NTSB recommendation for such crossings was installation of stop signs. That was seen as a far cheaper alternative to other railroad crossing safety measures, such as installing gates at a cost of $150,000 apiece, building bridges or rerouting tracks or roads.

Every state requires school buses to stop, turn off noisy equipment, open the doors and look both ways before crossing railroad tracks, according to the National Association of State Directors of Pupil Transportation Services, which supports the NTSB's recommendations.

But that does not always happen. The bus driver in the Tennessee crash never stopped at the crossing, even though it was marked with two warning signs and a crossbuck -- a white, X-shaped sign.

Research shows that while some know to "stop, look and listen" when they see a crossbuck, many others think the sign means "slow down" or that it does not require any special action, NTSB investigator Joseph Osterman said. There is no such confusion with a stop sign, which is why installing them at crossings makes sense, he said.

"It's a real easy solution," Osterman said. "Leaving an ambiguous sign that means different things to different people in different states is not a good answer."

Barbara Harsha, executive director of the Governors Highway Safety Association, agreed that crossings present danger but said she does not believe stop signs will persuade drivers to halt.

Beyond stop signs, the NTSB recommends equipping school buses with "option kill switches," which enable drivers to turn off noisy devices such as fans and radios when a bus approaches a rail crossing. Only Florida and Kentucky require the switches.

The NTSB also called for better training and oversight of bus drivers, including using video cameras to help monitor performance.

The rear of a bus sits several yards from the cab after it collided with a CSX train in March 28, 2000, in Tennga, Ga. Three children died in the crash.