For eight hours, Prince George's County firefighters barricaded a freight train in Beltsville because they believed the train to be leaking a toxic liquid. Then, at 4:30 yesterday morning, they made a reassuring discovery: the train was actually leaking water.

The train, which was carrying the chemical acrylonitrile, originally was stopped in Beltsville because the train's wheels lacked enough grease. While Chessie system railroad workers were examining the wheels, they found a leak.

They called in firefighters, Environmental Protection Agency officials and employes of American Cyanamid, the chemical company shipping the toxic liquid.

The firefighters arrived first, prepared for such a problem after having staged a train wreck near Beltsville in June. Two trains in the simulated wreck were actually carrying water. But in theory, one was carrying the flammable gas propane, while the other was carrying the toxic chemical chlorine.

Confronted last night by what they thought was a real emergency, the firefighters did some of the things they had rehearsed months ago -- erecting barricades around the train to prevent people from approaching the area and getting their hoses and pumps to extinguish any explosions.

Finally, after eight hours, chemical company employes arrived from New Jersey. They approached the freight train and tested the liquid.

After several minutes, a chemist announced his findings.

"It's water," he said.

Apparently, the water was from condensation, said Capt. John Scanlon of the county fire department.

"We had to go on the premise that it was the toxic substance," said Scanlon. "But we're relieved it wasn't."