Two freight trains collided Monday in a rural area, killing an engineer, derailing 40 cars and releasing clouds of chlorine and ammonium nitrate that drifted for miles.

About 50 people were hospitalized after being exposed to the chemicals, said Fire Department spokesman Randy Jenkins. The gas clouds dissipated by late morning, after drifting up to 10 miles to San Antonio (search)'s SeaWorld amusement park, where six people were treated for minor respiratory irritation.

Another six people had to be airlifted from the scene. The San Antonio Express-News reported that two of those injured were hospitalized in critical condition.

Sheriff's Deputy Chief Ray Trevino confirmed the engineer's death, but no other details on the fatality were available.

Search crews later found the bodies of two elderly sisters in two houses near the crash site, although officials would not say whether their deaths were related to the accident. The homes were about a mile from the railroad tracks.

"We're not necessarily making any association between this and the derailment," Jenkins said.

The collision occurred when a Union Pacific (search) freight train struck a Burlington Northern Santa Fe train, said Joe Faust, a spokesman for Fort Worth-based Burlington.

Union Pacific spokesman John Bromley said his company's train was traveling at about 20 mph at the time. He said 23 of the 74 Union Pacific cars derailed. Seventeen Burlington Northern cars derailed.

No evacuations were ordered, but two heavily traveled roads were closed.