Canadian Pacific Railway plans an investigation determine just what caused a train to jump the tracks south of Red Deer early Thursday, according to CPR spokesman Mark Seland.

About 20 cars of a 70-car freight derailed just north of the Springbrook, Alberta, level crossing. No one was hurt, but 28 residents of a nearby trailer park were moved out as a precaution.

One car that carried anhydrous ammonia appeared to be leaking, but was later found to contain only small amounts of the gas.

Residents were allowed to return to their homes Thursday afternoon. Work crews worked through the night to clear debris.

Seland says even though as many as 20 trains a day run between Calgary and Edmonton, a derailing on the main line is an uncommon occurrence.

The investigation will include a careful physical examination of the wreckage, and a review of information on electronic monitors along the line. All CP trains carry black boxes, he said.

CP already takes precautions to prevent such incidents, Seland said. "We look for hot wheels. We look for dragging equipment. Before any train leaves the yard it's visually inspected," he said. "We have event recorders on the train and our train crews are in constant communication with the network management centre so there's a lot of checks that look at preventing situations such as this."

It may be some time before an exact cause of the derailment is known.