Two runaway train cars carry military equipment from Fort Drum rolled about eight miles Monday before crashing into a CSX maintenance train and derailing, causing a small propane fire that prompted evacuations, authorities said.

One CSX worker suffered a sprained ankle trying to run from the accident. The cars crashed just after 9 a.m., forcing the temporary closing of nearby Interstate 81. Businesses and homes within a half-mile area were evacuated for about two hours as a precaution.

"It's a miracle this wasn't more serious," said Watertown Fire Chief Daniel Gaumont. "It's downhill from Fort Drum ... we believe these cars were going 40-45 mph."

Fort Drum spokesman Ben Abel said the two flatbed cars rolled away after 1st Brigade Combat Team soldiers finished loading them with equipment bound for the port of Philadelphia, and eventually Iraq. The 3,500-soldier brigade is scheduled to begin a 15-month deployment in September.

Army officials are investigating why the train cars broke free, Abel said.

He could not immediately say what they were carrying. Gaumont said a placard on the cargo containers cautioned that the contents included "flammable liquids, oxidizers and explosives."

Army officials had notified local authorities, who were searching for the runaway cars when the accident was reported, Gaumont said.

The collision caused the explosion of a 1,500 gallon propane tank stored on the CSX vehicle, igniting a small fire, Gaumont said. Firefighters allowed the fire to burn itself out, he said.

Hazardous materials teams from Watertown and surrounding communities were called to the scene because authorities were uncertain what substances they were dealing with, Gaumont said. The Fort Drum Mobile Incident Command Center also was sent to the scene.

"We were braced for the worse. It was very touch and go for a while," Gaumont said.